To efficiently diffuse heat generated following the increase in power by providing a film coated with ceramic coating and dried on a metal surface and setting the metal surface as the outside surface of a positive electrode.

A cylindrical positive electrode 11 is made of copper, and a black chromium plated film is formed to have a thickness of 4 to 5μm its inside surface by a wet method or the like. Tiny recessed and projecting parts are formed on the outside surface by blasting and, then, ceramic coating agents of one solution are coated on the outer surface of the recessed and projecting parts by spraying. The coating agents are dried and a film 11a is formed. Further, thermal treatment is provided during exhaustion, and one layer is dried and hardened. For coating agents, ceramic fine powders such as silica (SiO2) or oxides of iron and chromium (Fe3O4, Cr2O3) are main components. For binders, aluminum phosphate and coroidal silica are used, and these are dispersed in solvent such as water and ethanol for use.