PURPOSE: To permit easy and sure groove mating and to join satisfactorily all of multiple pipes by fitting high-strength sleeves into the stepped parts on the inside circumferential surfaces of the respective joint parts of the pipes facing each other and welding the outside peripheral parts with said sleeves as backing strips.

CONSTITUTION: A stepped part is provided on the inside peripheral surfaces in the contact part 3 between the machined part 1a of a header 1 and a coupling pipe 2 facing each other. A sleeve 6 which is a material having high strength is fitted thereto. The sleeve 6 acts as a backing strip for welding. Clearances B1, B2 assure satisfactory formation of a groove G even if there is minor error in assembly. The multiple pipes 2 are thus satisfactorily welded when the grooves G are welded from the outside periphery thereof.