The planetary gear train includes a first sun gear driven by the motor, a first ring gear, a first carrier, and a plurality of first planetary gears arranged at one side of the first carrier and meshing with the first sun gear and the first ring gear. The first sun gear, first planetary gears and first ring gear are helical gears so that meshing surface is increased. Helical teeth formed on an inner surface of the first ring gear mesh with the helical teeth of the first planetary gears. Therefore, the operation is stable and the noise is low. The planetary gears are made of plastic to further decrease the noise. The first ring gear is made by sintered powder metallurgy.