A ring gear (82) of a planetary gear speed reducer (80) is fixed to a casing (31) provided with a core metal (31) fixed to a vehicle panel via a rubber damper (92). Therefore, the driving unit can be reduced in size through adoption of a speed reducer (80) formed of gears (82) made of spur gears, and the rubber dumper (92) can suppress low frequency vibrations which are transmitted from the speed reducer (80) to the vehicle panel. Therefore, the vehicle body of the vehicle having a low characteristic frequency is prevented from resonating with the incoming low frequency vibrations to generate noises which are transmitted to the vehicle interior, and which give an occupant an unpleasant feeling.