To prevent a knife edge from being deformed by inserting a seal ring whose knife edge is formed along one end face, in an aluminum alloy except the knife edge holding and pressing a gasket on one end face, and making the whole of the knife edge higher in hardness than the aluminum alloy.

A seal ring 1 made out of stainless steel, whose knife edge 1a holding and pressing a gasket is formed along one end face, is inserted in an aluminum alloy by high pressure casting except the knife edge on the one end face of the seal ring 1. Thus as mentioned above, since the whole of the knife edge 1a is made out of stainless steel, the knife edge 1a will never be deformed even if fastening pressure is made high. For this reason, its vacuum sealing performance will never be impaired even if a flange is repeatedly mounted/demounted. Besides, the seal ring 1 is closely attached to the aluminum alloy 2, the boundary between them will never be delaminated even if the flange is heated up.