PURPOSE: To perform the fillet welding along a weld line at a high speed by detecting a welding current or the arc voltage of two welding wires and comparing the values to detect a deviation of torches from the weld line and sliding the torches to correct the deviation.

CONSTITUTION: The welding currents flowing between the tips of the respective welding wires 8 and 8', and base metal 1 are detected by ammeters 13 and 14 respectively the these detected currents are sent to a comparison computing element 15. The comparison computing element 15 compares the detected currents and in case the difference between both the detected currents is positive or negative, a slide motor 7 is rotated normally or rotated reversely by a normal and reverse rotation commanding device 17 to slide and move a sliding device 6. The welding torches 4 and 4' are moved in a direction to cross at right angles with the weld line 2 together with the welding wires 8 and 8' and stopped at the position where intervals between the tips of the respective welding wires 8 and 8' and the base metal 1 become equal. The tips of the welding wires 8 and 8' are advanced along the weld line 2 without weaving to perform the fillet welding at the high speed by performing the above-mentioned control continuously.