PURPOSE: To enable the speed change control with high efficiency by driving an output shaft consisting of a planetary supporting body through an internal gear forming a rotary body with two rows of planetary gears from an input shaft having the first and the second sun gears.

CONSTITUTION: When an input shaft 1 (the first and the second sun external spur gears 3 and 4) revolves at a prescribed speed, the first planetary external spur gear 5, the second sun external sour gear 6, and the third planetary external spur gear 7 rotate and revolve in the same direction to the revolution direction of the input shaft 1, and an output shaft 2 revolves, and a rotary body 8 (internal spur gear 9) revolves in the reverse direction to the revolution direction. When the load of the output shaft 2 relatively increases for the rotary body 8, each revolution of the rotary body 8 is accelerated as the load increases more, and each revolution of the first, second and the third planetary external spur gears 5, 5, and 7 is decelerated, and the output shaft 2 is decelerated. At this time, the output shaft 2 outputs the torque corresponding to the number of revolution. Accordingly, even in the low speed region, the sufficient torque can be outputted, and efficiency can be improved.