A motor shaft puts through a motor casing and a gear casing which are detachably connected to each other. In the motor casing, a rotor is fixed to the motor shaft, while a stator is fixed on the inner circumferential surface of the motor casing. The motor shaft has a worm which engages with a worm wheel in the gear casing. A collar in which the motor shaft is pressingly fitted is provided in the middle of the motor shaft at the end of the gear casing. The motor shaft is supported at each end of the worm in the gear casing.