PURPOSE: To prevent the generation of delamination cracks in the stage of fillet welding of rolled steel sheets, by melting beforehand deep the surface of the weld zone of the joint on the plane side and solidifying the same before execution of welding, placing the joint on the web side on said part and subjecting the weld zone to fillet welding.

CONSTITUTION: Plural pieces of arc welding torches 3 are disposed above the intended weld zone of a steel sheet 1 on the plane side of a tee joint. The steel sheet 1 in this case is beforehand tilted to 5W10° upgrade in the advance direction of the torches 3, and the plural torches 3 are disposed with slight deviation in the relative positions. While the torches 3 are advanced in this state, the intended weld zone 4 of the plate 1 is welded with the arc, and in succession, the welded part is quickly solidified to form uniform weld metal texture 5. The steel sheet of the joint on the web side is placed thereon and is subjected to fillet welding. The generation of delamination cracks is obviated even if the joint steel sheet contains about 0.030% S.