To provide a piston ring suitably used in a combination with a cylinder or a cylinder liner having a sliding surface structure reducing reciprocating friction.

The piston ring is formed in an outer peripheral sliding surface of the piston ring with a plurality of grooves which each extend from one end cutting out an outer peripheral line of either end in a ring axis direction of the sliding surface to the other end lying in a plane of the sliding surface and are spaced from each other in a circumferential direction of the outer peripheral sliding surface of the piston ring. The piston ring is combined with the cylinder or the cylinder liner formed with a number of recesses in the inner wall surface of the cylinder or cylinder liner.