PURPOSE: To enhance the durability of a piston ring having at least one of upper and lower surfaces, and inner and outer peripheral surfaces, formed thereon with a nitrided layer and at least one of the other surfaces formed thereon with a wear-resistant surface layer, by forming the above-mentioned surfaces on a nitrogen diffused layers.

CONSTITUTION: In the case of application to a side rail for a combined oil ring, a usual nitriding process is applied to the outer surface of a side rail material 14 formed from, for example, an SUS440B strip, and therefore, the outer surface is formed thereon with a black layer (not shown) consisting of a ζ phase and a phase, a white layer 12 consisting of a γ' phase and a nitrided layer 13. Then by a usual process the black layer is removed to expose the white layer 12 to the outer surface, and thereafter, the material 14 is made into closely contact with the cylindrical body 17, and with the repetition of this step a plurality of materials 14 are stacked one upon another. Then, the thus obtained assembly is dipped into a dilute hydrochloric acid to etch the outer surface of the white layer 12, and thereafter, a hard chromium plating layer 15 is coated on the thus exposed nitrogen diffused layer 13.