To suppress the occurrence of ruggedness on the surface and to suppress the cooling defects at the hardening portion in a hardness technique by using a high frequency induction heating for a thin-thickness conductive pipe material such as a steel-made golf shaft.

The thin-thickness pipe-shaped golf shaft W is lowered into a heating coil 210 to be subjected to the high frequency heating, and then the golf shaft W is dipped into a hardening oil 310 to be subjected to cooling, thereby hardning the golf shaft W. In that case, inert gas is supplied to the inside of the golf shaft W to prevent the hardening coil 310 from entering into the inner part of the golf shaft W. In this way, the hardening oil 310 is suppressed from boiling in the inner part of the golf shaft W, thereby preventing the blowing-up of the hardening oil 310 due to the boiling. Therefore, the uneven cooling caused by the blowing-up of the hardening oil 310 is prevented.