To install a gear pivoting pin pivoting a planetary gear with high precision.

A carrier is connected interlockingly to each of left and right central shafts 45L, 45R, and in the carrier, a plurality of pin insertion holes are bored at certain intervals in the circumferential direction. In each of the pin insertion holes, one side end part of a gear pivoting pin is inserted parallelly to the central shafts 45L, 45R, and in the other side end part of the gear pivoting pin, a notch groove is formed substantially orthogonally to the axial line of each gear pivoting pin. In each of the notch grooves, the outer circumferential edge part of a ring type stopper plate is inserted, while each stopper plate is fixed to each carrier. In this way, the respective gear pivoting pins are supported while held in their both sides, and left and right planetary gears are installed freely rotationally in the respective gear pivoting pins.