The arc current is divided into two areas each corresponding to a 1/4 period of weaving, the two areas are compared with each other, and an area corresponding to a predetermined time period at the end portion of the larger area is substrated from the larger area and another area comparison is performed. The movement of the welding torch is compensated in accordance with the predetermined correction function, so that, even when an actual weld line of base metals to be welded deviates from an instructed weld line, the welding torch is correctly moved along the weld line of the base materials and arc welding is performed.