PURPOSE: To improve the quality of weld zone by providing a moving element reciprocated by the magnetic field of a coil and a displacement detector to the rod supporting a torch thereby detecting the preset extent of movement, supplying electric current to the coil and reciprocating the torch to a groove width.

CONSTITUTION: When electric current is supplied to a stationary coil 12, a moving element 13 moves together with a rod 8 and a torch, toward the left. When the rod 8 reaches the preset extent of movement, the extent of movement is detected by a displacement output detector 15. Next, this signal is sent to a command signal generator 17 which in turn causes a change-over device 18 to actuate, thus the electric current having been supplied to the coil 12 is changed over to the coil 12' side. This causes the moving element 13 to move together with the torch 4 in the right direction. By repeating these operations, the torch 4 is reciprocated in the groove width direction.