The distance L₁₅ in the axial direction between the surface on the axially outside end of the spline shaft 28a and the opening edge on the axially inside end of the center hole 15 of the hub 5 when the unit is installed, and more preferable the distance L₁₆ in the axial direction between this surface on the axially outside end of the spline shaft 28a and the edge on the axially inside end of the spline hole 16, is greater than the distance L₁₈ that the tripod 21 of the constant-velocity joint 18 on the differential side can move inward in the axial direction when installed in the automobile. Therefore, it is possible to prevent the spline shaft 28 from falling out of the center hole 15 even when the connection member 3 is damaged, and to maintain the spline connection between the spline shaft 28 and the spline hole 16.