To provide a piston ring being easy to manufacture and capable of very effectively preventing the leaking of a fluid such as a blow-by gas.

The piston ring is formed in the shape of a partially cut ring and its closed gap 3 is widened and fitted over the outer peripheral surface of a piston. The fitting part 14 of an elastic material 5 is inserted in the closed gap 3 in the piston ring 1 which closes a gap that forms between the piston and a cylinder when the piston is fitted into the cylinder.