PURPOSE: To improve the yield and quality of a product by performing a swaging and extrusion on a metal material, forming a projection part at the inner side of an opening part by a drawing, then performing a forging formation via a punch and die.

CONSTITUTION: A metal material 1 attaining to a product piston is subjected to a swaging in a columnar body 1', then the bottomed cylindrical body 6 having a bottom part 3 at the end part 2 is formed with an extruding. As a 3rd stage the projection part 4 in the inner side direction is formed by performing a drawing on the opening part 7 of the cylindrical body 6. The product piston A having a flange part 5 is finally formed with cold rolling via the punch and die as a 4th stage. With this method a cut machining is eliminated and all the workings are performed with cold treatment, so the product quality including the yield and strength, external surface finishing, etc., of the product is improved.