PURPOSE: To perpendicularly move a torch, slew the same around the arc generating point and continuously weld the welding materials having waveform parts by detecting the clearance between the tip of the torch and the welding materials and the relative angle of the weld line and the torch.

CONSTITUTION: While the tip of a torch 2 held perpendicularly at a fixed spacing from a welding point B by a supporting member 4 is being moved along a guide rail 13 by a traveling carriage 11, it welds the plane part of welding materials 1. Next, when the welding point reaches the waveform part C of the materials 1, the clearance between the tip of the torch 3 and the materials 1 and the relative angle to the weld line are detected with sensors 14a, 14b. Then, the sliding part 8b of a backet 8 is moved upward, whereby the torch 3 is moved and is slewed around a slewing shaft 5. Here, the slewing constituting member of the torch 3 is so provided by being inclined to the angle not interfering with the surface of the materials 1 that the extension line of the centerline of the torch slewing shaft 5 passes the welding arc generating point, and therefore the torch 3 slews around the arc generating point 15 and welds the waveform part C.