PURPOSE: To form a weld zone having excellent strength and shape in the stage of butt-welding thin steel sheets, etc. with laser light by drawing out a flat belt- like filler metal to the butt part of said sheets and sandwiching the filler metal with the butt surfaces of the sheets then welding the butt part with the laser light.

CONSTITUTION: Thin steel sheet panels 13, 13 to be butt-welded are sandwiched by feed rollers 12, 12 constituting feed units 3A, 3B. On the other hand, a flat belt-like filler metal 7 wound on a roll 5 is passed through feed rollers 8 and chucks 9 and the rod 10a of an acutuator 10 on the opposite side is extended to grip the front end of the filler metal 7 by a mechanical hand 11 attached to the tip of the rod, then the rod 10a is contracted to introduce the metal 7 into the butt part 14 of both panels 13, 13. Feed rollers 12 are rotated to approach both panels 13, 13 and to sandwich the filler metal 7 then the laser light RE from a laser device 4 is moved along the part 14 and is irradiated to said part, thereby melting the metal 7 and welding the butt part 14 of the two panels.