PURPOSE: To eliminate likelihood of such inconveniences as drop of transmission efficiency, breakage of gear, etc., by furnishing No.2 driven wheel with a guide member, which is to prevent inclination of a planet gear caused by inclination of a rotary shaft.

CONSTITUTION: When power is transmitted to a driving gear 1, a motive wheel 2 rotates, and a sun gear 2a will rotate in turn. Consequently planet gears 6 meshing with the sun gear 2a will rotate. If at this time No.1 driven wheel 7 stops and the internal gear 7a thereof stops, the planet gears 6 spin round their own axes while orbiting round the sun gear 2a. A rotary shaft 12b consolidated with No.2 driven wheel 12 rotates round the sun gear 2a. At this time, breakage of gears and drop of transmission efficiency can be prevented by furnishing guide members 13 around the rotary shaft 12b in such a way as consolidated with No.2 driven wheel 12.