To raise reliability of inspection result and also to prevent occurrence of malfunction of an automatic transmission.

An automatic transmission 11 wherein an oil pump and a transmission device are built in an automatic transmission case 12 with an oil pan 31 fitted to a lower part of the automatic transmission case 12, a driving means for operating the automatic transmission 11, a tester for inspecting function of the automatic transmission 11 while it is operated, and an oil tank 41 which, connected to the automatic transmission case 11 through oil path L11 and L12, houses oil, are provided. The oil pump, according to behavior of the automatic transmission 11, supplies oil to the oil tank 41 or sucks it from the oil tank 41. Since the function of the automatic transmission 11 is inspected with the oil pan 31 fitted, reliability in inspection result is raised.