A welding method which has the steps of butting, for example, as shown in Figs. 3(a), 3(b) and 4, the edges of a pair of metal materials (3) to be welded together, such as steel plates, to form a through opening (10) such as an I-shaped opening formed by the gap therebetween, and feeding a filler wire (9) into the opening (10) for laser welding, thereby increasing and improving the allowable value of the gap of the through opening. More particularly, a laser beam (1) is focused by an optical system such as a lens or the like, the focus (11) of the beam (1) is positioned at the same level as the surface of the materials (3) above the opening (10) with a power density of more than 5 x 10 5 W/cm', and the materials are welded together by bringing the point of insertion of the wire (9) into the focus (11) of the beam (1) or into the focus (11) of the beam (1) on the upper surface of the wire (9) in the opening (10). in this case, the end of the wire (9) is melted by the laser beam to form a molten droplet which moves into the opening (10), thereby heating both side walls (12) of the opening and welding the materials together.