PURPOSE: To improve workability and simplify constitution by so constituting a forming wire whih is deformed in conformity with the oscillating motion of the welding arc that it is fed from a rotatable nozzle in front of a feeding mechanism provided to a welding torch.

CONSTITUTION: A feeding mechanism which feeds the above-described deformed forming wire 2' by sandwiching the same with a rotating belt 9 is provided to a welding torch 5 and electricity is conducted to this wire 2'. Also, a nozzle 13 guiding the wire 2' is provided in front of the feeding mechanism. The above-described feeding mechanism and nozzle 13 are made rotatable. The nozzle 13 is directed to the corner parts of the material 18, 19 to be welded intersecting at a right angle, by this semiautomatic welding device, thereby generating the welding arc 20 between the wire 2' which is being fed and the parts to be welded. When the wire 2' is fed continuously after this, the arc 20 automatically oscillates at the amplitude corresponding to the deformation width W of the wire 2. In this case, the operator is required simply to move the torch 5 along the weld line.