PURPOSE: To eliminate the engagement interference and obtain a super-compact transmission by increasing the transmission gear ratio by means of a set of planet gear mechanism including a frictional transmission mechanism, and providing the frictional transmission mechanism in place of a sun gear mechanism.

CONSTITUTION: An elastic ring 21 is fitted to a ring receiving body part 18 of the respective planet rotary bodies 20, and friction wheels 16 of the respective planet rotary bodies 20 are tightly pressed by this spring force against a high-speed rotary shaft 10 provided in the center. Thus, when the high-speed rotary shaft 10 rotates in the (F) direction, the friction wheel 16 is rotated in the (G) direction. And a gear 15 integrated with the wheel 16 is also rotated in the (G) direction, and the gear 15 engaged with the rotated internal gear 11 is revolved. This movement of the gear 15 allows a small-difference gear 19 integrated with the gear 15 to be rotated and revolved, and the internal gear 22 engaged with the small-difference gear 19 also to be rotated and revolved. The rotation and the revolution are cancelled by each other, and the internal gear 22 is rotated in an extremely reduced speed, obtaining an extremely reduced speed of a low speed rotary shaft 7 integrated with the internal gear 22.