To provide a piston ring for a piston reciprocating type compressor capable of automatically showing excellent sealing performance in any of compression stroke and suction stroke.

The annular piston ring is installed in a groove on a piston outer circumference surface and is slidably pressed on a cylinder bore inner circumference surface. A section shape of the piston ring includes a part having a shape in which angle D1 of a surface forming a part of the piston ring at high pressure fluid existing side in a cylinder to an cylinder bore inner circumference surface and angle D2 of a piston ring surface at a low pressure fluid existing side positioned at an opposite side of the high pressure fluid existing side with a part of the piston ring put therebetween to the cylinder bore inner circumference surface satisfy a relation D1D2, in both of compression stroke and suction stroke.