PURPOSE: To secure high airtightness and mechanical strength of a joint concurrently by fixing a member with an inserting head part on the top end of a first pipe material and a male screw member near the tail end of the member respectively it and also fixing the socket member of the inserting head on the top end of a second pipe material and a tapered member near the tail end of the socket member respectively.

CONSTITUTION: An inserting member 13 is fixed at the end of a tube 11, an inserting head part 14 is formed at the top end of the tube, and a male screw member 12 is fixed near the tail end of the inserting member 13 of the tube 11. Also socket member 23 provided with a socket 24 to receive the inserting head part 14 is fixed at the to end of the other tube 21, and a tapered member 22 provided with an outer peripheral tapered surface 22t is fixed near the tail end of the socket member 23. Then two projected parts 25 are installed elastically on the inner peripheral surface of the socket 24 so that they can be raised and sunk, and two recessed parts 15 capable of being fitted closely with the projected parts 25 are installed on the outer peripheral surface of the inserting head part 14. When inserting is advanced and the recessed parts 15 arrive at the projected parts 25, the projected parts are raised and sunk to fit closely with the recessed parts 15.