To prevent the falling off of a nut seat caused by a bolt load having a large axial force by attaching a taper roller to the tip of a cantilever shaft and turning this while pressing this to the nut seat and hardening the nut seat.

When a taper roller 4 is contacted with a nut seat 3, the taper roller 4 is turned while applying a pressing force produced in a spring 13 to the nut seat 3. At this time, a contact pressure is produced in the surface of the nut seat 3, the nut seat 3 and the structure of a back surface for backing up the same are pressurized and then meticulous machining and hardening is performed. When the vertex C of a conical surface forming the nut seat 3 is made coincident with that of the conical surface of the taper roller 4, the taper roller 4 is turned while not slid in the surface of the nut seat 3 and thus little 'peeling' is produced even if a contact pressure is increased and thus large pressed hardening is performed.