PURPOSE: To prevent a piston ring from breakage due to stick of sliding surfaces of rings by combining a plurality of rings bearing against each other on the surface approximately perpendicular to the axis or a piston.

CONSTITUTION: The thickness of a piston ring 2 is bisected by a plane perpendicular to the axis of a piston(not shown) into upper and lower rings 6,7 which are combined to have the cut-off openings 3a angularly spaced by 180° to each other. Then the upper surface of the ring 6 is formed concave at the outside 8 and the lower surface of the ring 7 convex at the outside 8'. When said concave surface fits said convex one, the length of engagement F is larger than a gap between a ring 2 and a ring groove(not shown) so that the rings 6,7 can ensure an integral operation. And by reducing the thickness B of the ring 2, the ring is prevented from breakage due to fatigue, since the substantial stress does not reach the repeated stress corresponding to the endurance fatigue limit due to the stick operation.