Abstract: A planetary gear mechanism 3 is provided at one end portion of a housing 2 in a direction of the rotation axis L. The mechanism 3 includes a planetary gear 31 disposed at the housing such that the planetary gear is rotatable about its own axis and revolvable about the rotation axis together with the housing, and an inner gear 33 and a sun gear 34 which are in mesh with the planetary gear. A differential gear mechanism 4 is provided at the other end portion of the housing. The mechanism 4 includes a casing 41 non-rotatably connected to the sun gear, a pair of pinion gears 43 disposed within the casing such that the pinion gears are rotatable about their axes and revolvable about the rotation axis together with the casing, and a pair of side gears 44, which are in mesh with the pinion gears.