a power supply configured to apply direct current power and to comprise a positive electrode terminal and a negative electrode terminal;

the multi-layered workpiece configured to be electrically connected to the negative electrode terminal to act as a negative electrode to which an electric current is applied, the multi-layered workpiece being disposed between a first base metal and a second base metal, the multi-layered workpiece being disposed in a narrow gap formed in a longitudinal direction between a first base metal and a second base metal facing the first base metal;

a torch having a positive electrode, which is spaced apart from the oxide film by a predetermined distance and is electrically connected to the positive electrode terminal, the torch being installed to be movable relative to the multi-layered workpiece;

a drive device configured to transport the torch in the longitudinal direction toward the multi-layered workpiece between the first base metal and the second base metal; and

a backing bar disposed adjacent to and beneath the multi-layered workpiece and the first and second base metals wherein an upper side surface of the backing bar is attached to the multi-layered workpiece and a lower side surface of the backing bar is electrically connected to the negative electrode terminal such that the backing bar electrically connects the work piece to the negative terminal,

wherein the oxide film formed on the multi-layered workpiece is removed by applying a reverse polarity direct current between the multi-layered workpiece, serving as the negative electrode, and the positive electrode to generate an arc,

wherein the torch comprises:

a torch body; and

a nozzle installed on the torch body to inject shield gas in a form of a laminar flow in the narrow gap between the first base metal and the second base metal, which are arranged adjacent to and facing each other, to be freely movable within the narrow gap groove, the nozzle having an elongated shape in the longitudinal direction, which is perpendicular to the upper side surface of the backing bar,

wherein each layer of the multi-layered workpiece is a weld bead layer generated by performing a gas tungsten arc welding in the narrow gap formed between the first base metal and the second base metal, the oxide film is formed on an upper surface of one weld bead layer of the multi-layered workpiece after the one weld bead layer is generated, and the oxide film is removed from the upper surface of the one weld bead layer by the applying of the reverse polarity direct current before a subsequent gas tungsten arc welding to generate a subsequent weld bead layer of the multi-layered workpiece is performed.