To provide the horizontal position MAG welding method capable of welding with high welding efficiently and low costs and obtaining an excellent bead appearance.

In a strap 4, a taper is provided in the corner part wherein the side surface of the welding progressive direction side and a lower plate come into contact and a recessed part is formed in the upper part facing a groove. This strap 4 is pressed to the upper end of the lower plate 2 so that the taper faces in the welding direction and arranged, a welding wire 5 is fed within the groove 3 from the top of the strap 4, and the strap 4 is slid in parallel along the welding direction as shown with an arrow to weld. At this time, the strap 4 is slid on the surface of the lower plate 2 in accordance with the welding progress while opening the upper half part of the side welding the groove. By this way, since welding is executed while the strap 4 always supports the molten pond, even though the upper half part of the side welding the groove is not covered with a copper plate or the like as the conventional way, the molten metal is prevented to overflow, whereby welding beads having the excellent appearance are obtained.