To enable effectively and accurately forming relatively small openings by forming preliminary holes in a plate material consisting of plastically deformative material before shear deforming the plate material in the surface direction and forming a plurality of formed holes each deformed into a long and slender shape.
A perforated material having a plurality of openings is formed of a plate material 1 formed of metal materials, such as stainless steel, iron, and copper. That is, in the plate material 1, a lot of lozenge-shaped preliminary holes are previously formed in prescribed arrangement, such as staggered arrangement by punching work, laser work, and etching work. The plate material 1 in which such preliminary holes 2 are formed is plastically deformed so as to be shear deformed in both directions, as shown by arrows A, B. In this way, a perforated material having slit-shaped formed holes 2a is formed. And such perforated material is stamped out in the suitable shape to form a product such as a fluid permeable wall material for holding a filter medium or particles.