To provide a method for positioning a planet gear to one or more ring gears and/or center shafts, and set a manufacturing tolerance of a gear and a hole part to be sufficiently large at a similar cost to that on a conventional epicycle reduction gear.

The epicycle reduction gear is provided with plural plane gears 7 having external teeth which rotate about a shaft, at least one ring gear 2, 10 having internal teeth, and/or a center shaft having external teeth. The shaft of the plane gear 7 is installed on a plane gear support body 8 to relatively move in radial and circumferential directions to the support body 8 in a perpendicular plane to a main axial line of the gear reducer, the shaft of the planet gear 7 is moved in the radial or circunferential direction to a position where the external teeth of the planet gear 7 are engaged with the internal teeth of the ring gears 2, 10 or the external teeth of the center shaft, and the shaft is fixed to the plane gear support body 8 at this position.