PURPOSE: To improve a working limit of a laser beam welding zone by preheating the vicinity of the end faces to be welded before laser beam irradiation to form the inside reinforcement and depressing a weld bead part to eliminate the reinforcement height after welding is completed.

CONSTITUTION: In pipe manufacturing welding, before a butt part is irradiated with a laser beam, the vicinity of the end faces to be welded is preheated above 250°C by a high-frequency induction method, etc., and the inside reinforcement height is formed to ≥0.15mm at least. After welding is completed, the weld bead part is depressed in the thickness direction by roll working, etc., and the formed inside reinforcement height is eliminated. Consequently, the welded steel pipe with satisfactory workability can be manufactured. It is desirable to eliminate the inside reinforcement height in a line at the same speed as welding. After the inside reinforcement height is eliminated, as necessary, at least the vicinity of the welding zone is subjected to heat treatment.