PURPOSE: To prevent the sticking of an inner pipe and an outer pipe in the case of butt-welding two double pipes provided respectively with the inner pipe and the outer pipe provided by being away from the circumference of the inner pipe at a specific distance.

CONSTITUTION: Before performing the welding, the joining face of one double pipe 1 of the double pipes 1, 2 on which a diffusion welding is performed is formed into an annular projecting face with the flow passage 9 as an apex, and the joining face of another double pipe 2 is formed into a recessed face to be fitted with the projecting face. Then, in the case of welding, the double pipe 1 is positioned lower, and simultaneously an insert materials 15, 16 are interposed between the inner pipes 3, 4 and between outer pipes 7, 8, thereby butting the double pipes 1, 2 in a perpendicular direction. In this state, the insert materials 15, 16 are heated while being moved from the lower part 15a, 16a towards the upper part 15b, 16b to prevent the insert materials 15, 16 from exuding to the flow passage 9, 10 on the outer circumferential side.