PURPOSE: To reduce input energy by transmitting input to an output gear and a first return gear through a first internal gear, two-stage gears, and a component force gear, and returning the power transmitted to the latter to the two- stage gears from an auxiliary gear.

CONSTITUTION: Rotational drive force is transmitted from a drive gear 6 to a second internal gear 3 so as to rotate it, through a first internal gear 2 and a supporting wheel 4. Rotation of the second internal gear 3 is transmitted to the large diameter side gears 10 of two two-stage gears, and the large diameter side gears 10 are revolved in the same direction as that of the second internal gear 3 through a fitting arm 9. Rotation of the small diameter side gears 11 is transmitted to a third internal gear 15, transmitted to a component force gear 21 through a supporting wheel 13, and simultaneously transmitted to an output gear 22 and a first return gear 23. Rotation of the output gear 22 is transmitted to an output transmitting means 28, power transmitted to the first return gear 23 is transmitted to a second return gear 26 and a third gear 27, an auxiliary gear 17 is rotated, and rotation of the small diameter side gears 11 is assisted. Thus, rotational load and current consumption can be reduced.