<p>An electric motor (1) and a worm gear (11) are received in a case. An output shaft (2) of the electric motor (1) and a fitting hole (11C) of the worm gear (11) are fitted to each other so that torque is transmittable between them and that they are relatively movable in the direction of an axis (X). A gap (D) in the direction of the axis (X) is formed between the forward end of the output shaft (2) and an inner end of the fitting hole (11C). A support member (30) in contact with an end of a shaft body (11A) of the worm gear (11), which end is on the electric motor (1) side, is received in the case. As a result, when the electric motor (1) is stopped, thrust force produced by dynamic inertia of the wheel gear (11) and acting on the worm gear (11) is received by the support member (30).</p>
