PURPOSE: To improve the installation efficiency and reduce the cost and to maintain the high strength and high corrosion resistance of the weld part intrinsic to the above double pipes by removing the inside pipe from one of the ends of the double pipes and the outside pipe from the other, forming a spacing between the outside pipe and the inside pipe of both, and then butt-welding the pipes.

CONSTITUTION: The double pipes 1 are constituted of the outside pipe 2 and inside pipe 3 which consist of metallic materials having the components different from each other. The double pipes 1' are similarly constituted of the outside pipe 2' and the inside pipe 3'. The inside pipe 3 at the end of one of the double pipes 1 is removed and the spacing 4 is formed between the outside pipe 2 and the inside pipe 3. The outside pipe 2' at the end of the other of the double pipes 1' is removed and the spacing 4' is formed between the outside pipe 2' and the inside pipe 3'. After the outside pipe 2 and the inside pipe 3 are beveled, the outside pipes 2, 2' and inside pipes 3, 3' of both the double pipes 1, 1' are butted against each other and are welded. Since the weld zone 5 of the inside pipe 3 and the root pass weld zone 6 of the outside pipe are parted in the axial direction of the double pipes, these weld zones do not come into contact with each other and since the spacing 4 is formed between the inside pipe 3 and the outside pipe 2, the pipes do not fuse to each other and the components of both of the weld zones 5, 6 are not diluted.