An electric motor includes a motor casing formed with a shaft insertion hole and a motor output shaft extending in the shaft insertion hole and projecting to a speed reducing mechanism. An annular oil seal is disposed between the shaft insertion hole and the motor output shaft and the annular oil seal includes an annular seal retainer buried in the annular oil seal for reinforcement. A C-shaped stopper member extending circumferentially from a first end to a second end spaced from the first end, is held in a holding portion or a fitting groove of the shaft insertion hole, and arranged to prevent movement of the oil seal toward the speed reducing mechanism. The stopper member includes a plurality of beam segments confronting the seal retainer from an axial direction of the motor output shaft.