PURPOSE: To prevent deposited metal from hanging down in performing butt welding of circumferential part in all positions by an arc welding machine using a nonconsumable electrode by applying current to a coil wound around the weld zone.

CONSTITUTION: A butt welding zone 18 of metallic tubes 10a, 10b is welded in all positions by an arc welding machine provided with a nonconsumable electrode 20 generating an arc 24. In this case, a coil 28 is wound around the tube 10a to be welded and current is started in the coil from an electric power source 30. When a welding torch makes welding in an area ED between 0hr position P0 and 6hr position P6, the direction of current is made reverse to the case where welding is made in an area EU between P6 position and P0 position. The welding arc 24 is deflected in the same direction with the direction of advance of welding by Fleming's left- hand rule due to magnetic line of force Φ in the direction of tube axis generated by welding current (i) and current I that flows in the coil 28. Hanging down of deposited metal is prevented by this arc force and thermal welding in all positions is made possible.