PURPOSE: To stabilize AC arc welding and prevent the occurrence of magnetic blow phenomena by welding the narrow groove part formed between high grade steels and themselves in a sealing gaseous atmosphere by flowing AC to a welding wire contg. a specific amount of an arc stabilizer.

CONSTITUTION: With the respective edge parts of steel plates S1, S2 prepared to a square groove held opposed specially, a backing strip 3 is abutted on one side face of the steel plates S1, S2 and a welding wire 1 is passed through a nozzle until its leading end faces the inside of the groove part. AC voltage is applied between the wire 1 and the steel plates S1, S2 or weld metal S3, whereby welding is executed. A composite wire filled with an arc stabilizer, a deoxidizer, etc. in a hollow body is used for the wire 1, and a material of high electron emission capacity such as metallic oxide, metallic fluoride or the like of 0.1W3% by weight of the wire is usable for the arc stabilizer.