An object of the invention is to always maintain an outer pin holder and a housing in a speed reducer section at their predetermined positions, to prevent damage to such components as revolving members, outer circumferential engagers, and motion conversion mechanism upon a large axial load due to turning or sudden acceleration/deceleration, etc., and to eliminate rattling noise caused by the housing and the outer pin holder. A casing immobilizing member which is fixed to an inner surface of a speed reducer section casing and a housing immobilizing member which is fixed to an outer surface of a speed reducer section housing are provided between the speed reducer section casing and the speed reducer section housing. The casing immobilizing member and the housing immobilizing member are bonded with each other via a rubber member.