To provide a method for manufacturing an internal-combustion engine forged piston capable of improving mechanical properties in a side of last solidification surface, reducing variation of mechanical properties, improving efficiency of machining process and quality by using an aluminum alloy forging material fabricated by one-way coagulation.

This method for manufacturing the internal-combustion engine forged piston includes a process for fabricating forging material by casting molten aluminum alloy with one-way coagulation casting method, a process for pre-heating the forging material, a process for forging the forging material to a piston workpiece having 70% or more modulus of plastic working on a piston head surface and a projection part on a center of the piston head surface after storing the forging material in a die with making the one-way coagulation last solidification surface side face to a surface of the die foe making the piston head surface, and a process for machining the piston workpiece to the internal-combustion engine forged piston.