It is intended to make it possible to, even in a situation where a program and/or data to be written to a memory device of an electronic control unit provided in an automatic transmission become different in content, depending on a specification of a vehicle to be mounted with the automatic transmission, manage the electronic control unit while distinguishing only by a difference in hardware configurations thereof. Disclosed is a method which comprises: an assembling step (10) of assembling an automatic transmission, wherein the assembling step includes installing an electronic control unit in the automatic transmission; and a testing step (30) of testing the automatic transmission after completion of assembling in the assembling step (10), wherein a writing step (20) is performed after the assembling step (10) and before the testing step (30), to write vehicle-dependent information consisting of a program and/or data depending on a specification of a vehicle to be mounted with the automatic transmission, to a memory device of the electronic control unit installed in the automatic transmission.