A welding system that, using a junction at which a standing plate and a bottom plate meet as a welding line, makes an electrode weave centered on the welding line and thereby welds along the welding line. In this welding system, control is performed such that the arc voltage at a standing-plate-side weaving edge is equal to or less than the arc voltage at a welding-line center position, such that the arc voltage at a bottom-plate-side weaving edge becomes equal to or greater than the arc voltage at the welding-line center position, and such that the arc voltage at the standing-plate-side weaving edge becomes lower than the arc voltage at the bottom-plate-side weaving edge. The system can suppress occurrence of inferior bead appearance and of welding defects in horizontal fillet welding.