PURPOSE: To simplify stages and to reduce the initial cost by forming the cut surface of a main pipe of a three-dimensional curve which is gradually changed in accordance with the outside diameter and wall thickness of a branch pipe, cutting the cutting surface of the branch pipe with a three-dimensional curve changing gradually in correspondence to the angle of inclination of the cut surface of the main pipe and butt welding the two pipes.

CONSTITUTION: The cut surface 2 of the three-dimensional curve, in which the side face shape recesses to the arc shape having the symmetrical faces at the cross section Z, is formed in at least the upper peripheral wall part 1a1 bordering at the horizontal neutral plane X of the main pipe 1 while the angle θ1 of inclination in the thickness direction of the peripheral wall part 1a1 is gradually changed in accordance with the outside diameter and wall thickness of the branch pipe 2 with respect to the cross section Z orthogonal with the horizontal neutral plane X and vertical neutral plane Y. The cut surface is formed in the peripheral wall part on the connecting end side of the branch pipe 3 by cutting this part to the shape projecting to the arc shape of the side face shape geometrically congruent to the cut surface 2 at the cutting angle gradually changing in correspondence to the angle θ1 inclination by the three- dimensional curve in the thickness direction. The cut surface 2 and the cutting surface are butted and welded against and to each other.