Abstract: The cylinder and piston of an injection pump which are severely corroded by molten aluminum or the like are made of a composite sintered body containing two or more of the compounds selected from the group consisting of boron carbide, titanium diboride, zirconium diboride and boron nitride, and having excellent corrosion resistant, wear resistant and heat shock properties and high mechanical strength. One or more of the compounds selected from the group consisting of borides of tantalum, molybdenum and tungsten; carbides of silicon, zirconium, tantalum, vanadium,chromium, tungsten and molybdenum; nitrides of titanium, aluminum, silicon and zirconium; and oxides of aluminum and beryllium may be incorporated.