To reduce the demand for dimension accuracy and assembling accuracy of respective parts by connecting at least one of inner races to an input shaft, fixing either one of retainers in such a state as not revolving, connecting the other retainer to an output shaft, and constituting outer races, inner races, a rotating body, and the retainers in differential relation.

When force is input in an inner race 10 of the first traction drive 1, a ball 12 rotates by keeping a constant position, because a retainer 13 is fixed, so that this rotation movement is transmitted to an outer race 11. The first and second traction drives 1, 2 are integrated with each other at respective outer race 11, 21 parts and inner race 10, 20 parts and this rotation rotates and revolves the ball 22 so as to be transmitted to the retainer 23. The outer races 11, 21, inner races 10, 20, balls 12, 22, and retainers 13, 23 are in differential relation so that the decelerated rotation is output to the retainer 23 and this output is taken out coaxially with the input.