A planetary gear unit inside a case includes coaxial sun and ring gears, a planetary pinion gear that meshes with the sun and ring gears, and a carrier that holds the planetary pinion gear, the ring gear being fixed with respect to the case, and a flange disposed inward of the ring gear. The ring gear is fixed to a case wall, via the flange, the wall extending radially inward, and the flange being prevented from rotating with respect to the wall by a rotation prevention mechanism. The rotation prevention mechanism includes a plurality of case holes on the same circle in the wall, flange holes in the flange at positions corresponding to the case holes, and rotation prevention members fitted into the case and flange holes to prevent rotation of the flange with respect to the wall with the flange relatively movable axially with respect to the wall.