PURPOSE: To connect a branch joint to a main pipe without using welding or brazing, by opposing the distal end of the branch joint against an attaching hole in a first nut, and by screwing a second nut onto a second thread.

CONSTITUTION: A first thread 16 having a large diameter is formed in a main joint pipe 4 which is attached to a main pipe 1 and which surrounds a pressure receiving seat surface 3' in the pipe 1. A first nut 7a having a large diameter is fastened onto a first thread 16, being opposed to the pressure receiving seat surface 3' in a gas-tight manner. This first nut 7a is axially formed therein with an attaching hole 20 in which a second thread 21 is formed. A branch pipe 5 is surrounded by the inner periphery of the attaching hole 20, being opposed against a press head part 6 of a press seat surface 17. A second nut 25 is inserted onto the branch pipe 5, and is screwed onto the second thread 2 so that a conical press seat surface 17 of the press head part 6 is opposed against a conical surface 22 of the attaching hole 20, thereby it is possible to secure the branch pipe 5 to the main pipe 1.