Consolidated Balance Sheet
| Concept | As at 2024-06-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2023-09-30 | As at 2022-10-31 | As at 2022-10-31 | As at 2022-10-31 | As at 2022-10-31 | As at 2022-10-31 | As at 2022-10-31 | As at 2022-10-31 | As at 2022-09-30 | As at 2022-09-30 | As at 2022-09-30 | As at 2022-09-30 | As at 2022-09-30 | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Consolidated balance sheet | ||||||||||||||||||||||||||||||
| Balance sheet | ||||||||||||||||||||||||||||||
| Assets | ||||||||||||||||||||||||||||||
| Current assets | ||||||||||||||||||||||||||||||
| Cash and deposits | 
                                                2,055,371,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                86,884,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                1,549,069,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Notes receivable, accounts receivable from completed construction contracts and other | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                493,866,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Contract assets | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                202,541,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Costs on construction contracts in progress | 
                                                37,429,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                63,653,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Other | 
                                                233,349,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,750,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                61,625,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Current assets | 
                                                2,943,172,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                88,635,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                2,371,206,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Non-current assets | 
                                                376,012,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,165,647,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                367,829,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Investments and other assets | 
                                                140,931,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,164,757,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                133,225,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Deferred tax assets | — | — | 
                                                2,776,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                77,140,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Other | — | — | 
                                                5,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                12,562,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Allowance for doubtful accounts | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                -44,170,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Non-current assets | ||||||||||||||||||||||||||||||
| Property, plant and equipment | ||||||||||||||||||||||||||||||
| Buildings and structures | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                59,998,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Accumulated depreciation | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                -16,309,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Buildings and structures, net | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                43,689,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Own-used assets | ||||||||||||||||||||||||||||||
| Machinery, equipment and vehicles | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                77,467,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Accumulated depreciation | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                -65,563,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Machinery, equipment and vehicles, net | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                11,904,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Land | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                119,094,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Other | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                6,949,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Accumulated depreciation | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                -5,826,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Other, net | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                1,123,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Property, plant and equipment | 
                                                194,250,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                890,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                184,896,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Intangible assets | 
                                                40,830,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                49,707,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Goodwill | 
                                                40,553,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | 
                                                49,243,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                49,243,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                49,243,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Other | 
                                                277,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                464,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Intangible assets | ||||||||||||||||||||||||||||||
| Other intangible assets | ||||||||||||||||||||||||||||||
| Investments and other assets | ||||||||||||||||||||||||||||||
| Assets | 
                                                3,319,184,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,254,283,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                1,888,098,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                720,253,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                130,683,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                2,608,351,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | 
                                                2,739,035,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Liabilities | ||||||||||||||||||||||||||||||
| Non-current liabilities | ||||||||||||||||||||||||||||||
| Retirement benefit liability | 
                                                104,773,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                110,160,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Other | 
                                                18,260,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                19,160,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Non-current liabilities | 
                                                123,033,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                218,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                129,320,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Current liabilities | ||||||||||||||||||||||||||||||
| Short-term borrowings | — | — | 
                                                30,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Income taxes payable | 
                                                120,123,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                10,636,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                75,728,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Provisions | ||||||||||||||||||||||||||||||
| Provision for bonuses | 
                                                113,302,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                4,896,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                62,194,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Accrued expenses | — | — | 
                                                3,240,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                39,570,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Asset retirement obligations | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
                                                17,150,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Other | 
                                                195,725,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                14,626,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                115,103,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Current liabilities | 
                                                922,815,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                86,825,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                672,311,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Provision for bonuses for directors (and other officers) | 
                                                30,148,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                7,844,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                18,713,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Liabilities | 
                                                1,045,848,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                87,044,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                801,632,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Liabilities and net assets | ||||||||||||||||||||||||||||||
| Shareholders' equity | 
                                                2,273,335,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,167,238,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                1,937,403,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Share capital | 
                                                88,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                88,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                88,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Capital surplus | 
                                                52,605,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,073,976,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                52,605,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Retained earnings | 
                                                2,132,730,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                5,262,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                1,796,798,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Net assets | 
                                                2,273,335,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,937,403,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,167,238,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                52,605,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,796,798,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,073,976,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,167,238,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | 
                                                88,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                5,262,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,937,403,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                88,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                5,262,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,073,976,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,689,598,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                108,802,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,548,993,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,689,598,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                31,802,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Liabilities and net assets | 
                                                3,319,184,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,254,283,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | 
                                                2,739,035,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Net assets | ||||||||||||||||||||||||||||||
| Shareholders' equity |