PURPOSE:To prevent strain and to improve the resistance to fatigue of a metallic material by subjecting the desired part of said material to a heat treatment while applying the elastic prestrain in the direction opposite from the strain generated in the metallic material after the heat treatment thereof on said material. CONSTITUTION:A round bar 1 is supported at fulcrums 2 in the positions equidistant from the center in the longitudinal direction thereof and a strain gage 3 is mounted on the bottom surface corresponding to the heat treatment part 1a on the top surface of the bar 1 between said fulcrums. Thereupon, load P is applied at both ends of the bar 1 to apply the elastic prestrain in the direction opposite from the strain to be generated after the heat treatment of the bar 1. For example, a laser beam L is irradiated to the part 1a in this state, whereby said part is heat-treated.
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