Abstract: Each of two coupling members is formed in an abutting end lace thereof with a recessed portion for accommodating a gasket and a retainer therein, and the recessed portion has a retainer holding hollow cylindrical portion formed centrally on a bottom surface thereof and provided on an end face thereof with an annular projection for clamping the gasket. The gasket comprises a small portion and a large portion having a greater outside diameter than the small portion. The retainer is attached to the cylindrical portion of the coupling member with which the gasket small portion is in contact.