To effectively transmit torque by supporting shaft directional reaction generated between respective balls and a groove of a disk plate in the vicinity or on its outside of a diameter of a crest of the groove continuously arranged in the circumferential direction of the disk plate.

A support center diameter D1 of shaft directional reaction generated between respective balls 5 and a groove 3a of a disk plate 3, is set in the same as a diameter D2 of a crest of the groove 3a continuously arranged in the circumferential direction of the disk plate 3. Therefore, respective end parts 3e of an outside diameter of the disk plate 3 are not deformed even if thrust force is applied since rigidity is secured, contact of the balls 5 and the groove 3a becomes perfect, and torque can be effectively transmitted. Abrasion of the balls 5 and the groove 3a is also reduced, and durability is improved. When the reaction indicating center diameter D1 is set larger than the center diameter D2 of the crest of the groove 3a, its effect is improved further.