An object of the present invention is to maintain a predetermined positional relationship between an outer pin holder and a housing in a speed reducer section B, 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, and to eliminate rattling noise caused by a housing and the outer pin holder. An elastic member is disposed on one or both axial end surfaces of the outer pin holder, between the outer pin holder and the housing. As a result, even if there is an axial load exerted on the outer pin holder, the outer pin holder is always held at a predetermined position by restoring force from the elastic member(s), in a proper positional relationship with the housing.