To prevent the wear of a pinion while facilitating work for mounting the pinion on an output shaft by increasing the axial tooth size of the pinion to be mounted on an output shaft of a motor.

The pinion 34 is externally fitted and mounted onto the output shaft 28 of the motor 16 via splines 37, 42. A fitting groove 38 is formed in the outer peripheral face of a base portion 28b of the output shaft 28 while extending in the peripheral direction. An elastic body 39 in an O-ring shape is provided which is fitted into the fitting groove 38 and whose outer peripheral portion is protruded from the outer peripheral face of the base porion 28b outward in the radial direction. An engaging groove 43 which can elastically engage with the outer peripheral portion of the elastic body 39 is formed in the inner peripheral face of the pinion 34.