A ring gear (82) of a deceleration mechanism (80) is fixed to a casing (31), and the casing (31) is provided with a metal core member (91) that is fixed to a vehicle body panel via a rubber damper (92). Thus, size reduction can be achieved by adopting the deceleration mechanism (80) which is formed, inter alia, by gears (82) constituted by spur gears, and in addition, the propagation of low-frequency vibration from the deceleration mechanism (80) to the vehicle body panel can be suppressed by the rubber damper (92). Thus, the vehicle body panel, which has a low natural frequency, is prevented from resonating, and it is possible to reliably prevent noise accompanying the resonance of the vehicle body panel from propagating to the inside of the vehicle and causing discomfort to occupants of the vehicle.