A piston of an internal combustion engine, comprises a skirt portion in sliding-contact with a cylinder wall, an inner crown-plus-boss portion having a crown portion and piston-pin boss portions, and a stay portion interconnecting the skirt portion and the inner crown-plus-boss portion at a lower portion of the piston. Also provided is an annular partition groove through which a rim of the skirt portion and the inner crown-plus-boss portion are partitioned all around the circumference of the upper portion of the piston. A thermal-expansion absorption ring is tightly fitted into the annular partition groove in a gas tight-fashion. The top face of the inner crown-plus-boss portion, the flat face of the rim of the skirt portion, and the top wall of the thermal-expansion absorption ring serve as a piston crown.