PURPOSE: To enable the scale-down of the outside diameter of an engaging shaft and downsize a bearing by forming a shaft hole for an output shaft, with its output side as an open end, at the center of a rotor, and dislocating the position of the bottom of the shaft hole at the other end and the position of the engaging part of a bearing arranged on that side so that they may not overlap each other in axial direction.

CONSTITUTION: A shaft hole 23 is made, with its output end side open, at the center of the core 21 of a rotor 20, and the position of the bottom 25 of the shaft hole 23 is arranged to lie on the opening side more than the base of the the engaging shaft 24a where a ball bearing 26a is set, and the shaft hole 23 is not made within the section where the ball bearing 26a is set of the engaging shaft 24a. Hereby, it becomes possible to make the outside diameter of the engaging shaft 24a small, consequently, a small-sized ball bearing 26a can be used. Furthermore, by making it have stopper function by abutting the end face 31a of the output shaft 30 against the bottom of the shaft hole 23, the outside diameter of the engaging shaft 24b can be made small, too, and the ball bearing 26b can be downsized, too.