An electric motor 1 and a worm gear 11 were housed in a case, an output shaft 2 of the electric motor 1 and a fitting hole 11C of the worm gear 11 were fitted so as to be capable of freely transmitting torque and of freely moving relatively in a direction of an axis X, and a tip of the output shaft 2 and an inner end of the fitting hole 11C set a gap D of separation in the direction of the axis X. Furthermore, a support section 30 making contact with an end section of a shaft section 11A of the worm gear 11 at the side of the electric motor 1 thereof was provided inside the case, and a thrust force resulting from dynamic inertia of the wheel gear 11 upon stopping of the electric motor 1 and acting on the worm gear 11 was borne by the support section 30.