A gear housing accommodates a worm shaft. A brush holder includes a base member and a retaining member loosely fitted to the base member. The base member is located between the gear housing and the retaining member. The retaining member retains a first brush and a second brush. The retaining member has a pair of positioning poles, and the gear housing has a pair of positioning projections. The positioning poles and the positioning projections are engaged with each other so as to prevent the retaining member from being moved in a direction perpendicular to the axial direction of the rotary shaft.