A constant velocity universal joint shaft, capable of suppressing a rotational backlash when incorporated in the constant velocity universal joint, and improving the assembly efficiency as well as reducing the manufacturing cost, is to be provided. The present invention relates to the constant velocity universal joint shaft having a spline that fits an inner race of the constant velocity universal joint. The shaft includes a shaft body with a spline formed thereon, and a yoke portion with a ring-shaped fitting portion. The shaft body and the yoke portion are integrally formed of medium carbon steel. By a pressing force of a pressing member the fitting portion of the yoke portion is tightened, so that such fitting portion fits a shaft external to the joint. In the constant velocity universal joint, the pressing member and an abutment member are elastically butted via an elastic member, so that a ball constantly remains in contact with a raceway of an outer member and a raceway of an inner member.