Abstract: The loose-flange type fitting according to the present invention is closely sealed with an elastic gasket whose both the sides and outer circumferential face are pressed against a pipe collars and a reinforcing ring so that the seal contact therebetween increases in degree together with rising of a tubeside internal pressure, in which the seal material comprising an annular elastic gasket, a metallic reinforcing ring disposed on the outer circumferential side of the elastic gasket and the inner circumferential face of the elastic gasket having substantially a V-shaped cross-section and the axial width of the gasket being broader than that of the reinforcing ring under non-compression conditions.