PURPOSE: To form the skirt part of a piston having a large extruding ratio while using aluminum alloy for piston stock, further, to make the burr forming to whole periphery of edge of skirt uniform over whole periphery.

CONSTITUTION: In the forged piston forging die provided with a male die 4 and female dies 2, 3, a load stripper 12, which locates above a female die 2, is fitted in the periphery of male die 4 free to slide and suppresses flow of alloy stock in forging, is provided, a load stripper is of cylindrical, shape, auxiliary pressurizing devices 11, 11 are arranged above a load stripper 12, a heating member is integrated in a male die 4 and female die 2. By this method, Flow of alloy stock is evenly generated over whole the periphery of skirt part of piston as well as uniform burr is formed over the whole periphery of skirt edge part, crack is not generated, the forged product of piston not having unformed part is produced.