Disclosed is an internal combustion engine's piston with a piston head having first and second thinnest portions that are thinnest in thickness of the piston head. With respect to the center axis of the first and second pin holes for supporting the piston pin, the first and second thinnest portions of the piston head are respectively provided on both sides of a transverse axis that is perpendicular to the center axis of the first and second pin holes. Each of first and second rib portions that are respectively formed on inner surfaces of first and second pin boss portions extends at the piston head in a direction along the transverse axis to overlap each of the first and second thinnest portions of the piston head. This piston is capable of relaxing stress concentration at a position above the piston pin.