To provide a piston for an internal combustion engine with reduced friction in sliding a skirt and weight, and improved strength.

An area of the skirt 13a at a thrust side of a pair of skirts is larger than an area of the skirt 13b at a non-thrust side, a circumferential width of the skirt 13a at the thrust side is symmetrically gradually widened from its upper end to its lower end, and the skirt 13b at the non-thrust side has a constant circumferential width from its upper end to its lower end.