Patent ID: 8550956

Claim:
A planetary gear unit housed inside a case and comprising a sun gear, a ring gear disposed coaxially with the sun gear, a planetary pinion gear that meshes with the sun gear and the ring gear, and a planetary carrier that holds the planetary pinion gear, the ring gear being fixed with respect to the case, wherein: the planetary gear unit includes a ring gear flange disposed on an inner peripheral side with respect to the ring gear; the ring gear is fixed to a case wall via the ring gear flange, the case wall being formed in the case to extend radially inward, and the ring gear flange being prevented from rotating with respect to the case wall by a rotation prevention mechanism; and the rotation prevention mechanism includes a plurality of case holes formed on the same circle in the case wall, flange holes provided in the ring gear flange at positions corresponding to the case holes, and rotation prevention members fitted into the case holes and the flange holes to prevent rotation of the ring gear flange with respect to the case wall with the ring gear flange relatively movable in an axial direction with respect to the case wall.