In order to secure a contact angle between the inner surfaces of the inner and outer surface side spline grooves 28, 29 and the rolling surfaces of the balls 30, 30 to thereby secure the rigidity and load capacity of the ball spline 16 in the rotation direction thereof, the spline grooves 28, 29 must be formed in such a manner as to have a section of such a Gothic arch shape as shown in FIG. 13 ex