PURPOSE: To simplify stages and to reduce the initial cost by cutting away the peripheral wall part of a thick main pipe to a valley-shaped recess, reducing the width of the cut surface formed in such a manner from the rear side in accordance with the thickness in the crest-shaped peak part at the connecting end of a thin branch pipe and butting and welding the valley-shaped recess and the crest-shaped peak part against and to each other.

CONSTITUTION: At least one peripheral wall part 1a1 of the neutral plane X of the main pipe 1 is cut away to a side face valley shape to form the valley- shaped recess 2. The peripheral wall part at the connecting end of the branch pipe 3 is then cut to form the crest-shaped peak part 4. The width of the valley- shaped cut surface is reduced by back chamfering K1 from the rear side of the one peripheral wall part 1a in accordance with the thickness of the branch pipe 3. The crest-shape peak part 4 of the branch pipe 3 is then mated with the valley-shaped recess 2 of the main pipe 1 and the cut surface and the cutting surface are butted and welded against and to each other, by which the branched pipe is produced. The stages are, therefore, simplified and the initial cost is reduced. The degradation in the strength and the hermetic sealing by the deterioration of the coupled part with time is obviated.