PURPOSE: To completely and uniformly conform an abutting surface and the whole surface of a downward truncated conical surface with each other to prevent a welding defect by countersinking a hole having a size based on the internal diameter of a branch tube formed on the flat part of a main tube to the abutting surface conforming with the downward truncated conical surface of the branch tube.

CONSTITUTION: The flat part 2 having an area where a branch tube 3 can be provided vertically is formed by a press on the circumferential wall part 1a1 of the upper side of a horizontal neutral surface X of the circumferential wall part 1A of the main tube 1 and the hole 2A is formed on this flat part 2 in accordance with the internal diameter of the branch pipe 3. Thereafter, this hole 2A is countersunk to form the abutting surface 2a which agrees with the downward truncated conical surface 4 of the branch tube 3 and the width of the abutting surface is equalized to the downward truncated conical surface 4, then, the branch tube 6 can be manufactured by using an automatic or a manual TIG welding equipment 5. Consequently, since the manufacturing process is simplified and no manufacturing equipment on a large scale is required, manufacturing equipment cost can be reduced drastically.