A magnetic field is generated in a workpiece in a direction orthogonal to the joining direction, and as a result of the Lorentz force resulting from the magnetic field and the current between the plasma torch and the workpiece, the front tip side of the arc is bent forward in the direction of advancement of the plasma torch and welding is performed. The magnetic field strength of the welded part is adjusted by changing the relative positions of the plasma torch and a butting portion of the workpiece.