A drive mechanism for an automatic rear-view mirror assembly of a motor-driven folding type. The driving mechanism includes an electric motor fixed on a mirror housing. The mirror housing is supported rotatably on a shaft fixed to a mirror base which is secured to a vehicle body in such a manner that it can be turned between a normal and retracted position by the drive mechanism. The drive mechanism includes a motor and a gear reducer composed of a plurality of planetary gear units. The planetary gear units include a sun gear connected to an output of the motor, and a group of transmission gears including a first spur gear mounted fixedly on the shaft and a second spur gear connected to an output shaft and meshed with the first spur gear. The reduction gear includes a first casing which houses the planetary gear units on the same shaft and has a ring gear common to these planetary gear units, provided on its inner circumferential surface. The reduction gear also includes a second casing which houses the remaining planetary gear units on the same shaft as the planetary gear units of the first unit, so that the planetary gear units of both casings are meshed with each other. The second casing also has a ring gear on its inner circumferential surface and a bolt by which the first and second casings are combined unitarily with each other. The ring gears of the first and second casings are formed with different gear modules, with the ring gear of the first casing being formed with a smaller gear module than that of the ring gear in the second casing.