To reduce fatigue stress around a connecting part on a branch pipe side by making a diameter expanding part by swelling the vicinity of a tip part on the branch pipe side when the branch pipe is connected with a through- hole of a main pipe, making a tip part continuous to this diameter expanding part as a straight cylinder part fitted and inserted into the through-hole and joining engagement parts each other by welding.

When a branch pipe 2 is welded or adhered to a through-hole 1-2 punched on a peripheral wall part of a main pipe 1, the vicinity of a tip part of the branch pipe 2 is swollen and an integral type tapered part 21-1a is formed. A tip part continuous to this diameter expanding part 2-1a is made as a connected head part 2-1 constituted of an abutting surface 2-1c which is abutted on an outer peripheral wall part of the main pipe 1 and a straight cylinder part 2-1b fitted and inserted into the through-hole 1-2. The straight cylinder part 2-1b is inserted into the through-hole 1-2 and the abutting surface 2-1c is connected with an outside face of the main pipe 1 by welding or adhering. By this connecting structure, fatigue stress applied to a joining tip part of the branch pipe 2 can be reduced and a vibration-proof fatigue characteristic can be improved.