PURPOSE: To cause no blow hole nor pit on a weld zone and to obtain the weld zone having high reliability in the strength and aritightness as well by forming the dry thin film of water glass on the surface of the weld zone of two members to be welded and by performing a laser beam welding by transmitting the thin film thereof.

CONSTITUTION: A water glass 11 is coated so as to cover a butt line 10. In this case, the water glass is coated over the fixing base 9 on which two plates 7, 8 are lined not only run along the butt line 10. The water glass is solidified with its drying and the transparent thin film having the strength linking itself and the adhesive force to the plates 7, 8 and fixing stand 9 is formed. Then when a laser beam is irradiated from the upper vertical part and moved along the butt line 10, the butt welding is performed with the melted metal being enclosed under the thin film and the dense weld zone without flow hole and pit is obtd. This welding is applied for the welding place of a low melting point metallic material and high steam pressure metallic material.