PURPOSE: To enable welding with a higher quality without lowering productivity by making a welding torch travel simultaneously with cutting of the tip part of a succeeding sheet metal, and preheating cut parts in the butt welding of a preceding sheet metal and a succeeding sheet metal.

CONSTITUTION: A preceding sheet metal 1 is held by an outlet side clamp 4, a lower blade 6 is allowed to ascend, and the preceding sheet metal 1 is cut by a travel blade 5 on the upper side. Next, the lower blade 6 is allowed to descend, a succeeding sheet metal 2 is inserted into an inlet side clamp 3, and the tip part of the succeeding sheet metal 2, is inserted into the lower part of the preceding sheet metal 1. An inlet side frame 15 during a standby, keeping its distance from an outlet side frame 16, is allowed to go ahead, and a clamp interval is reduced. The succeeding sheet metal 2 is held by the inlet clamp 3, the lower blade 6 is allowed to ascend, and the succeeding sheet metal 2 is cut by the travel blade 5 in the same position as that of the preceding sheet metal 1. At this time, a welding torch 8 is allowed to travel from the rear of the travel blade 5, and a cutting part is preheated by a laser beam. And then, the lower blade 6 is allowed to descend, a backing metal 21 is allowed to ascend, and a laser beam welding torch 8 is allowed to travel to perform butt welding of the cut part.