PURPOSE: To harden a pipelide metal body while minimizing appearance of a strain by circulating cooling water through the hollow support part of a jig supporting the inner wall of the body to cool the body.

CONSTITUTION: Worked body 11 is heated to a predetermined temp., lid 18 is attached to inductor 14, and cooling water 15 is continuously injected into the lower part of hollow support part 13 through cooling water introduction pass 16. Water 15 overflows from part 13 and is prevented from flowing toward the upper part of part 13 by lid 18, so water 15 flows into gap 17. The end of body 11 is cooled sufficiently and rapidly with circulating cooling water 15, and water 15 is discharged out of body 11. Since the inner wall of body 11 is closely contacted to part 13, body 11 is surely positioned and held, and appearance of a strain is reduced. Accordingly, treatment before or after hardening can be made unnecessary.