A hollow shaft material is provided with, on an outer peripheral surface side thereof, for example, a movable induction heating coil wrapped therearound. While a high-frequency current with a predetermined frequency is caused to flow through the induction heating coil, the induction heating coil is axially moved to perform induction hardening of the hollow shaft material from the outer peripheral surface side. At this moment, the frequency of the high-frequency current caused to flow through the induction heating coil is set relatively low as to the small-diameter portions, which have a relatively small thickness, and the frequency of the high-frequency current caused to flow through the induction heating coil is set relatively high as to the large-diameter portion, which has a relatively small thickness.