To carry out an uniform processing by preventing a ring gear from flexure in the radial direction.

By projectingly providing an annular guide member 10 made concentric with an axis in one side face in a body frame 1, by slidably fitting an annular guide groove 7 in the side face of the ring gear 5 in the peripheral direction to this guide member, and also steps 11, 12 formed in inner and outer peripheral end portions in the other face of the ring gear are retained by regulating a slidable, axial and radial movements with two press rings 14, 15 of the inside and outside fixed and equipped in the body frame 1.