To adjust the sliding resistance of a piston accurately by providing a clearance on a ring of resin material at one position in circumferential direction and forming axial and circumferential grooves in the internal surface of the ring in the sliding force stabilization ring provided between the piston and a cylinder.

A sliding force stabilization ring 1 is provided with a ring body overall of which is formed with a resin material such as fluoroethylene resin or polyacetar resin. Also a clearance 11 is provided in the ring at one location in circumferential direction so as to provide a spring effect to the ring 1. Then an axial groove 12 extending in the axial direction of the ring 1 and a circumferential groove 13 along the circumference of the ring 1 is formed in the internal surface of the ring 1. By providing the axial groove 12, a tightening force generated when the ring 1 is installed on a piston can be uniformed easily and, by providing the circumferential groove 13, lubricating oil can be accumulated in the groove so as to reduce a variation in sliding resistance between the piston and the ring 1.