PURPOSE:To apply a diamond film or a diamondlike carbon film excellent in mechanical strength, furthermore improved in wettability and affinity to aluminum, in which the prolongation of the service lives of a member for aluminum Molten metal, a member for machining and a member for plastic working such as extrusion are attained, moreover excellent in adhesion and capable of withstanding practical use. CONSTITUTION:The surface of a WC-based cemented carbide is subjected to boronizing treatment to remove away metallic components, by which its affinity and wettability to aluminum are improved and the prolongation of the service lives of a member for aluminum molten metal and a member for plastic working is attained, and moreover, on the surface, a diamond film or a diamondlike carbon film extremely excellent in adhesion can be formed.