A socket is attached to an end of one pipe and a plug is attached to an end of another pipe. The socket is provided with a pair of notched grooves at peripheral faces on both sides opposed to each other and a stopper constituting a U-shape is mounted onto the notched groove. The plug is provided with a first taper portion, a flat portion, and a second taper portion continuously from a front end side thereof in a sectional shape thereof along an axial direction, and a groove having the stopper inserted therein is formed at a ridge portion of the second taper portion. An inner periphery of the socket is arranged with a seal ring for sealing an interval between the inner periphery of the socket and an outer periphery of the plug in an airtight connection.