A thin-walled connecting pipe for pipeline, having an annular protuberance formed at a predetermined distance from a pipe end, the annular protuberance having an outer side face serving as a disengagement preventing face approximately perpendicular to the pipe axis, and the outer peripheral surface of a straight pipe portion on a front end side with respect to the annular protuberance serving as a seal ring abutting surface. Also disclosed is a coupling structure for such thin-walled connecting pipe, the coupling structure comprising a socket and an annular nut, the socket having a seal ring for abutment with the seal ring abutting surface of the pipe and a recess for receiving the annular protuberance of the pipe therein, the annular nut having a fore end face for abutment with the outer side face of the pipe and a flange for abutment with an outer end face of the socket, the nut having an inside diameter equal to the pipe diameter and being fitted on the pipe in threaded engagement with internal threads formed on the inner peripheral surface of an outer end portion of the socket adjacent to the recess of the socket.