PURPOSE: To exceedingly suppress deformation and arbitrarily adjust the strength at each part by applying the shot peening onto the dispersed places and adjusting the density of the dispersed places.

CONSTITUTION: The strength of a plate-shaped metal material 1 is increased by applying the shot peening work by projecting shot particles at the dispersed places A corresponding to the circular opened holes 3 formed on a pair of shielding members 2a and 2b positioned on both the sides of the plate-shaped metal 1. Further, each different desired compression stress can be applied onto each part of the plate-shaped metal member by adjusting the density of the dispersed places. Therefore, the strengthening corresponding to the use of the plate-shaped metal material 1 can be carried out.