A hard film is covered on the outer circumferential surface of a piston ring by means of an arc ion plating. The hard film has a mixture of crystal structure of CrN and crystal structure of Mo.sub.2 N containing oxygen in a solid solution state, and comprises chromium at 40 to 78 percent by weight, molybdenum at 3 to 35 percent by weight, oxygen at 3 to 20 percent by weight and the remainder of nitrogen. The crystal particle size is less than 1 .mu.m. The Vickers hardness is within a range from 1600 to 2500. The film thickness is preferably 1 to 60 .mu.m. Carbon in a solid solution state at 0.5 to 8 percent by weight may be substituted for the oxygen. Alternatively, both oxygen and carbon may be contained in a solid solution state with oxygen at 3 to 20 percent by weight, carbon at 0.5 to 8 percent by weight with the total content of carbon and oxygen within 25 percent by weight.