At the abutting end faces of coupling members (1, 2), annular recesses (3, 4) having annular protrusions (5, 6) on the bottom face are provided while surrounding fluid channels (1a, 2a). A gasket (10) comprises a seal part (11) interposed between the protrusions (5, 6) of the coupling members (1, 2) while having an outside diameter smaller than the diameter of the annular recesses (3, 4), and a guide part (12) disposed on the outside of the seal part (11) while having an outside diameter being fitted in the annular recesses (3, 4). When the coupling members (1, 2) are fastened properly, the end faces thereof abut each other. An annular groove (14) is made in the outer circumference of the seal part (11) and a snap ring (13) fitted in the annular groove (14) couples the seal part (11) and the guide part (12).