PURPOSE:To prevent the formation of the weld butt part of extremely thin- walled sheet materials as a result of heating and to execute safe welding by restraining locally the butt part by tack welding then subjecting the same to normal welding. CONSTITUTION:The extremely thin walled sheet materials 10 which are base metals are butted to each other by a suitable positioning and fixing means. The materials are then locally tack-welded along the butt part 12 by a welding and heating means 14 (plasma arc, high pulse arc, etc.) having an extremely high degree of focusing. The tack welding is thus executed in the extremely narrow width without generating the deformation as a result of heating the butt part 12. The butt part is then welded by a suitable normal welding means 16, by which the weld zone having high quality is obtd.