PURPOSE: To strengthen a plate-shaped metal material free from deformation by projecting shot particles onto a number of dispersed places arranged along the directions X and Y and applying each compression stress at the dispersed places and setting the residual stress in an intermediate region between the contiguous dispersed places along the arrangement direction, different from the compression stress at the dispersed places.

CONSTITUTION: Each dispersed place A is applied with a compression stress by projecting shot members onto the dispersed places A arranged along the directions X and Y in correspondence with the circular opened holes 3 in the shielding members 2a and 2b positioned on both the sides of a plate-shaped metal material 1. Further, the residual stress in the X direction in an intermediate region between the contiguous dispersed places A along the arrangement direction is set as the compression stress larger than the compression stress in the X direction at the dispersed place A, and the residual stress in the Y direction in the intermediate region is set as the compression stress smaller than the compression stress in the Y direction at the dispersed place A. Therefore, the plate-shaped metal member 1 can be strengthened free from deformation.