Abstract: A flared end structure 24 formed by processing an end part of a metal tube 20 is pressed against a seat 25 formed in a member 21 by tightening a coupling nut 27 to the member 21. The flared end structure 24 has a joining end part 30 to be pressed against the seat 25 of the member 21, and a curved part 32 continuous with the joining end part 30. The curved part 32 has an outer surface 31 having a curved area, and a concave inner surface 34 having a bottom edge 34a. The curved area of the outer surface 31 has a radius R of curvature smaller than the wall thickness t of the metal tube 20.