Abstract: An outside member connected to a first shaft is provided on the inner peripheral surface with three grooves extending in the axial direction of the first shaft at circumferentially equal intervals. An inside member connected to a second shaft has roller shafts extending outward in the radial direction of the second shaft so as to respectively enter into the grooves of the outside member, each roller shaft being provided on the outer periphery with a convex spherical surface. A cage provided on the inner periphery with a concave spherical surface is mounted on each roller shaft by fitting the concave spherical surface to the covex spherical surface of the roller shaft. A ring-like roller is rotatably supported by the cage to contact with the outside member. When the first shaft and the second shaft are rotated with a joint angle made between the first and second shafts, the position of the roller relative to the axis of the first shaft is maintained constant.