A second housing is aligned with a first housing at a predetermined position using positioning means that determines the mounting position of the housing with respect to the first housing at the predetermined position; steering shaft supporting sections and output shaft supporting sections are provided in the first housing and the second housing respectively while the housings being mounted using screws or the like; the second housing is removed temporarily from the first housing; a steering shaft and an output shaft are supported using the steering shaft supporting sections and the output shaft supporting sections; and the second housing is aligned again with the first housing at the predetermined position using the positioning means and mounted.