PURPOSE: To increase the average squashed load by a heat treatment, to control the increase of the squashing pitch without increasing the strength as the whole, and to stabilize the squashing mode, by setting the heat treatment part in a specific scope from the tops of the corners while leaving a nontreating part between the corners.

CONSTITUTION: In an effective width (a) from the tops of the corners K to both sides H of a high strength member 10, a heat treatment is applied locally and the strength is increased at the heat treatment part N shown as oblique lines more than the other parts. The heat treatment part N is formed by setting laser beams in an effective width (a), for example, so as to radiate the corners K. As a result, the heat treatment is carried out locally efficiently by limiting the scope. The effective width (a) is set as a=4. nt0.46 (where t is the thickness of the plate, and n is the number of corners) or higher. Consequently, an impact energy is absorbed mostly by the plastic deformation of the scope of the effective width (a) from the tops of the corners K, so as to increase the average squashed load Pave, and a large energy can be absorbed securely.