To provide a coupling method to prevent the occurrence of a trouble when flow division pipes different in diameter are relatively displaced.

At an expansion process, the vicinity of the slit groove 22c of a second pipe 20 is expanded and caused to approximately coincides with the diameter of a first pipe 10. At a consecutive covering process, the end part of a second partition plate 22 is covered with a covering member 30 having an arcuate recessed part 31 recessed in an arcuate shape with high precision. At a consecutive coupling process, a first and second pipes 10 and 20 are relatively displaceably intercoupled through a ball joint. In this case, an end part 12b on the first pipe side and an arcuate recessed part 31 on the second pipe side are arranged with a given distance (d) therebetween, and arranged such that both of them form the circular arc of a concentric circle centering around a point Ce. Therefore, even when the second pipe 20 is rotated around the point Ce and displaced due to vertical vibration of a vehicle, the arcuate recessed part 31 is displaced with the given distance (d) kept between the arcuate recessed part and the end 12b.