PURPOSE: To eliminate the need for anti-corrosive treatment after punching by increasing clearance between a punch and a die and forming so that a flange part is made a ruptured part.

CONSTITUTION: The clearance C between the punch 21 and the die 23 is set for 1/2 of the thickness of a steel sheet 12. The steel sheet 12 with the surface treated for anticorrosion is punched by the press so set, and the curved flange part 13 is formed at the punched edge. In the process of forming this flange part 13, it is also accompanied with the anti-corrosive treatment 11 of the steel sheet. Then, the ruptured part 14 is approximately flush with one surface 12a of the steel sheet 12. The flange part 13 stays covered with the anti-corrosive treatment, and the surface area of the ruptured part 14 is so small that it is negligible.