To couple a pipe body with a diameter change or bending in the vicinity of a tip part to be coupled without welding.

A tip part 16a of a flange 10 and a tip part 20a of a pipe 20 are inserted into an inserting part 40a and an inserting part 40b, respectively, force is capplied from the inside of an adapter 30, a small diameter part 36 becomes a large diameter and the diameter part 36 is swollen and deformed so as to be smaller as the diameter goes closer to both end part sides and also the tip parts 16a, 20a are swollen and deformed to the outside. By swelling and deformation, the tip part 16a and the tip part 20a are flared and the tip parts 16a, 20a are in close connection with an inner pipe 32 and large diameter parts 38a, 38b. As a result, a neck part 16 and the pipe 20 are surely and strongly coupled via the adapter 30. Because force is applied from the inside of the adapter 30, the outer peripheries of the neck part 16 and the pipe 20 are not required to be held by hardware and can be coupled, even if a diameter change such as a bowl-shaped part 14 exists near the tip part 16a.