To lighten and miniaturize a piston and form a piston for a high-speed revolution. A top ring groove and an oil ring groove are formed on the periphery of a crown and a lubricator is provided inside the oil ring groove. A pair of piston pin bosses are provided continuously under the crown in opposite positions and a skirt is provided apart by 90°. The lower end of the skirt is located on the upside of the lower end of the piston pin boss and a lubricator is provided in an upper part of the skirt, under and in the vicinity of the oil ring groove.