PURPOSE: To reduce a load on a motor and to improve an output efficiency by a method wherein a second internal gear fixing board at which an output means and a second internal gear having a diameter lower than that of an output means is integrally and concentrically fixed is attached, and a planetary gear is brought into internal contact with and engaged with the second inner gear.

CONSTITUTION: When a rotational drive force is applied from a first internal gear 10, planetary gears 18 and 19 are rotated and operated to rotate a stationary gear 21. Since the stationary gear 21 is not rotated, as a result of reaction being applied on the planetary gears 18 and 19 from the stationary gear 21, the planetary gears 18 and 19 follow movement of the first internal gear 10 and are revolved as they are rotated around the stationary gear 21. Through rotational and revolving movement of the planetary gears 18 and 19, a second internal gear 20 smaller than the first internal gear 10 engaged therewith and effecting free rotation is rotationally driven in the same direction as revolution directions of the planetary gears 18 and 19. Revolutions of the second internal gear 20 is transmitted and outputted to a sprocket 26 for an output, being an external output means, through a second internal gear fixing board 22 as it is.