To increase productivity and to enhance the quality of a weld zone by enclosing the tip end of the last electrode of multiple electrodes with a coil, imparting to it an alternating current in a specific hertz range, welding while forming in the tip end of a welding wire a magnetic flux density of a specific gauss, and thereby improving the welding speed of submerged arc welding of a steel tube.

Alternating current of 5-15 Hz flows from the power source 11 of an alternating magnetic field to the coil 3 wound between the steel-made inner and outer skins. The magnetic filed having 50-400 gauss of magnetic flux density at the tip end of a welding wire is generated. Since the magnetic field is produced by winding an enamel copper wire in the circumferential direction of the fourth electrode 15, and the magnetic field 25 is obtained by the coil in the axial direction against a welding electrode on the steel-made inner skin 2 side of a magnetic stirring coil. Meantime, a welding arc 18-21 for the first to fourth electrode is generated while a proximity to a weld zone is covered with a flux 17 for submerged arc welding between the welding wire and the base material from each center through the first to fourth electrode 12-15.