PURPOSE: To always supply a welding wire to the accurate position without exchanging the wire nozzles each time by providing plural wire nozzles conforming to each wire diameter according to a kind of the welding wire diameter and operating a wire nozzle selecting device.

CONSTITUTION: The plural wire nozzles 3A and 3B having the different inside diameters mutually corresponding to the welding wire 4 diameter are provided. In a state with the wire nozzle 3A selected, the welding wire 4 sent out from a spool 5A by a wire sending-out device 6A is led by the wire nozzle 3A via a wire feed tube 11 and supplied to a beam spot from the welding torch 2. When a command to the effect that materials 1 to be welded of a next stage are changed is inputted, the wire nozzle selecting device 12 is operated. A table 16A for ascending and descending is first raised and a table 13 for changing over is moved crosswise up to the position where the wire nozzle 3B can be selected and then, the table 16A for ascending and descending is lowered up to the specified position. In this state, a wire sending-out device 6B is operated to send out the welding wire 4 from a spool 5B. The welding wire 4 is led by the wire nozzle 3B via the wire feed tube 11 and supplied to the beam spot from the welding torch 2.