PURPOSE: To obtain a highly efficient manufacture of a welded steel tube by welding with a laser beam, a high density energy beam, tilted in a specific angle range to the side where welding is completed and thereby preventing the generation of undercut, blowhole, burn-through, etc.

CONSTITUTION: The end faces of a steel strip are abutted on each other while being formed into an open pipe, and then, the abutting part 2 of the open pipe stock 1 is irradiated for welding with a laser beam 3 which is converged by a machining lens and which is tilted at an angle θ to the side where welding is completed. In this case, the tilted angle θ of the beam is the one formed by the optical axis 4 of the laser beam 3 and the normal 5 at the irradiation point of the beam, bringing in a satisfactory result only in the case of tilting in a prescribed angle range to the side where welding is completed. This angle range is 5°≤θ≤20°.