To stabilize arc welding in which feeding of a welding wire is altered between forward feeding and reverse feeding periodically. The welding is performed in a manner of alternating the feeding of the welding wire between the forward feeding and the reverse feeding periodically according to a feeding rate pattern of a feeding-rate setting signal Fr constituted of a forward feeding amplitude Ws, a reverse feeding amplitude Wr, a forward feeding period Ts and a reverse feeding period Tr, stored in correspondence to an average feeding-rate setting signal Far, and generating short-circuiting periods and arc periods. An average value of a feeding rate Fw during the welding is detected, and at least one of the forward feeding amplitude Ws, the reverse feeding amplitude Wr, the forward feeding period Ts or the reverse feeding period Tr is changed so that the average value of the feeding rate Fw equals the average feeding-rate setting signal Far, and thereby the feeding rate pattern is automatically corrected. Consequently, even if a feeding resistance of a feeding path changes, as the average value of the feeding rate Fw is always maintained at a constant value, stability of a welding state can be improved.