PURPOSE: To secure a press-fit margin in the tooth thickness direction so as to prevent play in the rotating direction, and reduce a press-fit load in the case where an outer spline of a male member is press-fitted to an inner spline of a female member.

CONSTITUTION: One of a tooth side surface 3a of an outer spline 3 and a tooth side surface 4a of an inner spline 4 is formed into a curve while the other is formed into a curve having a curvature different from the former or plane shape. A press-fit margin α in the tooth thickness direction is partly secured between the tooth side surfaces 3a, 4a. Consequently, it is possible to limit a region where the tooth side surfaces bite each other, so as to reduce a press-fit load.