A sliding surface of a high-temperature portion is subjected to an electro-discharge surface treatment with one or both of a high-temperature hard material (4) and a material having a lubricating property at a high temperature (6). The high-temperature hard material (4) is any or a mixture of cBN, TiC, TiN, TiAlN, TiB.sub.2, WC, Cr.sub.3C.sub.2, SiC, ZrC, VC, B.sub.4C, Si.sub.3N.sub.4, ZrO.sub.2, and Al.sub.2O.sub.3. The material having the lubricating property at the high temperature (6) contains chromium and/or chromium oxide (Cr.sub.2O.sub.3) and/or hexaboron nitride (hBN). An electrode formed by compression molding of the high-temperature hard material, and the high-temperature lubricating material containing at least one of Cr and hBN and having the lubricating property at the high temperature is used as the electrode for the electro-discharge surface treatment.