A system that prevents the engagement of an intermediate gear and a driven gear from releasing due to a force from the driven gear without increasing an arrangement angle is described. A surface receiving a load acting on a rotation axis of a second groove that is used as an axle bearing formed on the side of a second gear part that engages with a driven gear is made to be an inclined receiving part. The second receiving surface touches the rotation axis of the second groove part. The surfaces are arranged such that the distance from the center of rotation of the driving gear increases as it moves away from the driven gear. By this arrangement, the inclined receiving part creates a force in the opposite direction of the direction to release the intermediate gear from the driven gear that acts upon the rotation axis as the counteraction of the force that acts on the rotation axis, so the intermediate gear does not separate from the driven gear.