Eight torque transmission balls (3) are arranged separately. A ratio r1 (=PCDBALL/DBALL) of a pitch diameter PCDBALL (PCDBALL=2xPCR) of a torque transmission ball (3) to a diameter DBALL is set to a value in the range of 3.5</=r1</=5.0. A ratio r2 (=DOUTER/PCDSERR) of an outer diameter DOUTER of an outer coupling member (1) to a pitch diameter PCDSERR of a serration (or a spline) (2c) of an inner coupling member (2) is set to a value in the range of 2.5</=r2<3.2.
