Abstract: A method of manufacturing a piston of an internal combustion engine includes holding a soluble core to a plurality of first projection portions and a plurality of second projection portions and a plurality of third projection portions formed in a pair of straight portions of a strut member for preventing thermal expansion of the piston. The first projections have a bent portion bent outwardly so as to be along an inner face of the soluble core and the second projection portions have a curved holding portion bent inwardly so as to be along a lower surface and inner surface of the soluble core. By holding the strut member and the soluble core in the piston, the soluble core is melted away and an oil-cooling gallery is formed by pouring water into the mold.