Since the compression raschig ring is provided in only its outer peripheral surface
with a plasma nitrided layer, the abrasion resistance with respect to the sliding contact with the cylinder is good. Since the upper and lower surfaces and an inner peripheral surface are base surfaces or are formed with soft surface treatment layers, the abnormal abrasion of the raschig ring groove of a piston is hard to occur. When a soft surface treatment layer is formed, the good anticorrosion is obtained.