To perform a process for forming an inclined plastic deformation part in an axial end part of a spline tooth at low cost.

When a spline tool 39 is inserted into a spline hole 36a and then drawn out by applying torque Q to a spline shaft 39, the axial L end part of the spline tooth 36a is plastically deformed and the plastic deformation part 36b inclined to the axial line L is formed. Thus, at the time of press fit of the spline shaft 39 and the spindle hole 36a, the plastic deformation part 36b serves as an interference and looseness of a fitting part can be prevented. Since only by changing the magnitude of the torque Q and drawing out speed of the spline tool 39, the length and angle of the plastic deformation part 36b can be easily changed by means of only one type of the spline tool 39, various types of spline holes 36a can be processed without preparing form rolling racks with respect to each type, enabling drastic reduction in processing cost.