PURPOSE: To improve durability of a belt by forming a drive pulley, which transmits power to a driven pulley by a toothed belt, in a planet gear mechanism, in which the drive pulley is eccentrically mounted to a rotary shaft, and setting a length of the belt in a manner wherein an eccentric position is placed in a belt loosening side.

CONSTITUTION: When drive power is given to a rotary shaft 1 of a drive pulley 3, a carrier 4 is rotated, and the drive power is transmitted to a ring gear 6 through pinions 8 from a shaft part 5 further to a driven pulley 14 through a toothed belt B. Here following the drive power given to the carrier 4, centers of the ring gear 6 and a sun gear 7 are moved with a belt loosened side left as placed in an eccentric condition for a straight line which connects the center of the rotary shaft to the center of the driven pulley 14. Thus by increasing a pitch peripheral length of the toothed belt B to eliminate its looseness, proper tension can be given to the belt. By the constitution in the above, generation of damage in the toothed belt or the like, is suppressed and its noise reduction can be contrived.