PURPOSE:To obtain a weld zone having no welding defect by placing two pieces of electrodes in the width direction of a narrow groove, executing alternately an electric conduction and deflecting an arc in the narrow groove width direction, and oscillating the arc in the narrow groove. CONSTITUTION:An arc 8 is generated from a tungsten electrode 2 of the right side, but the arc 8 is deflected to a groove side wall surface 16 of the right side by a magnetic field which has been generated by an electric conduction to a filler wire 11. Also, the arc 8 which has been generated from a tungsten electrode 3 of the left side is deflected to the groove side wall surface 16 of the left side. In this way, the arc 8 is oscillated to the right the left and in a groove 15, and also inclined and deflected to the corner part of the groove side wall surface 16 and a weld metal 13, therefore, the groove side wall surface is melted stably. In this case, it will suffice that the oscillation of the arc 8 controls electrically only the electric conduction to the tungsten electrodes 2, 3, and also the filler wire 11 is supplied near the center between the electrodes 2, 3.
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