surface 11 to engage the small spheres fitted in the small sphere receiving holes 6 1 6 2 6 6 of the inner rotary body 1 for transmitting a rotational force through these small spheres a connecting part 9 is adapted to be connected with the other one of the input side rotating shaft or the output side rotating shaft because the center of the concave surface forming the inner surface 11 of the outer ring part 10 of the outer rotary body 8 is generally inside the outer ring part 10 on the rotational axis of the outer rotary body 8 the diameter of the edge part of the inner surface 11 in the direction perpendicular to the rotation axis is smaller than the diameter of an assembled inner rotary body in the direction perpendicular to the rotation axis and it is therefore impossible to insert the inner rotary body 1 into the outer ring part 10 of the outer rotary body 8 after the small spheres are fitted into the respective small sphere receiving holes 6 1 6 2 6 6 of the inner rotary body 1 as a first step therefore a pair of small sphere receiving holes 6 1 6 4 located at positions diametrically facing each other are selected from the small sphere receiving holes 6 1 6 2 6 6 of the inner rotary body 1 and small spheres 7 1 7 4 are forced into the pair of small sphere receiving holes 6 1 6 4 in the radial direction respectively.