PURPOSE: To prevent leakage of exhaust gas by arranging welding cut portions previously at adjoining end portions of respective exhaust pipes and forming welding portions for preventing leakage of exhaust gas in the cut portion thereby preventing cutting of the anti-leakage welding portion through cutting after welding.

CONSTITUTION: Respective end portions 2 of an exhaust pipe 1 are arranged laterally in a row, and an oblique cut portion 16 is formed at one adjoining end portion longitudinally from endface. The cut portion 16 is formed to have such depth as the deepest portion is positioned farther outside by a distance (l) than the final finish inner diameter L of the case 5 when the valve case 5 is coupled with the exhaust pipe 1. When adjoining portions are welded in the cut portion 16 to form welding portions 16 for preventing leakage of exhaust gas, end portions 2 of four exhaust pipes 1 are integrated 4. The integrated body 4 is fitted on a coupling port 6 then entire circumference is welded to the valve case 5 so as to integrate them. Thereafter, a valve 8 is matched to have inner diameter of L, and since finish face 17 is in the way of the cut portion 16, only tip side of the welding portion 3 is cut partially.