A joint driving apparatus for a robot comprises an oscillating internally meshing reduction mechanism, a center shaft member, a ring portion, and an oil seal disposed between the ring portion and the center shaft member. The oscillating internally meshing reduction mechanism is fixed to a first member and rotates the second member relatively with respect to the first member. An output member is arranged at the side of an external gear in an axial direction so as to output a relative rotation of an internal gear with the external gear. The center shaft member is fixed to the output member, and has a shaft portion extending up to the side opposite to the external gear in the axial direction of the output member. The ring portion is integrally formed with the first member and faces to the shaft portion of the center shaft member. The oil seal is disposed between the ring portion and the shaft portion.