PURPOSE: To smoothly rotate a sun gear and a planet gear by providing a first teeth part and a position adjustment projection in the part of the periphery of the sun gear of a same axis center and loosely fitting the planet gear to the rotary shaft of a planet arm by providing a backlash part.

CONSTITUTION: The first teeth part 53 and the projection 54 for adjusting positions are provided in the part of the peripery of the sun gear 52 freely rotatably supported by the rotary shaft 56 and also the planet gear 58 provided with a second teeth part 59 in the periphery is loosely fitted to the rotary shaft 61 of the planet gear arm 55. In this case, the backlash part 60 is provided by making the central diameter of the gear 58 larger than the diameter of the shaft 61 and also a stopper 57 regulates the position of the arm 55. Thus, when the gear 52 is rotated in the direction of an arrow, the both peaks of the projection 54 and the teeth part 59 contact by pressing and when it is rotated further, the gear 58 moves to an A direction and the gears 52 and 58 are rotated. At this time, the backlash part 60 prevents the excess loads of the projection 54 and the teeth part 59 and the projection 54 moves the teeth part 59 to a prescribed position. Thus, the teeth parts 53 and 59 are smoothly engaged and the generation of damage and noise is prevented.