The tetrafluoroethylene-perfluoroalkylvinyl ether copolymer is prescribed in ASTM D 3307, and has two types, i.e. Type I and sort II. The tetrafluoroethylene-hexafluoropropylene copolymer is prescribed in ASTM D 2116, and has 4 types, i.e. Type I to Type IV. It is preferable that the elastomer