To improve volumetric efficiency in a compressor, and to reduce a lubricating oil discharge quantity, by improving sealability by a piston ring.

This piston of the compressor is provided by installing the piston ring having a main constituting material composed of resin in a piston ring groove. The piston of the compressor is characterized in that roundness or chamfering is applied to a cross-sectional shape viewed in the piston longitudinal sectional direction to a corner part on the anti-piston top surface side among corner parts between the piston ring groove and a piston outer peripheral surface adjacent to its both sides, and a curved surface capable of abutting on a part to which the roundness or the chamfering of the corner part is applied, is arranged in a cross-sectional shape viewed in the piston longitudinal sectional direction in the piston ring.