PURPOSE: To perform flare welding extending over the entire length of the rib by fitting an auxiliary member having the same spiral shape as the rib and having the length of ≥ one pitch to a welding finishing side end of the rib and welding the rib to a steel pipe.

CONSTITUTION: The spiral rib l is welded and joined to the inside of the steel pipe. The auxiliary member 2 having the same spiral shape as the rib l and having the spiral, length of ≥ one pitch is then fitted attachably and detachably to the welding finishing side end of the rib l. The rib 1 is then welded and joined to the steel pipe. Consequently, floating of the spiral rib is prevented and welding can be performed suitably.