PURPOSE: To move a torch so as to make differences zero and accurately control the torch to the groove center by detecting the back pressure of a shielding gas changed by the deviation of the torch and comparing the same with a reference signal.

CONSTITUTION: When a welding torch 3 moving in the welding advance direction deviates to either one of the groove width directions A, B, a change in the back pressure of a shielding gas 5 owing to a change in the shapes of welding materials 1, 1' is detected by a gas pressure detector 8. At this time, the reference signal corresponding to the gas pressure in the groove central part 2' is outputted from a reference signal generator 10. Next, the detection signal and the reference signal are compared in a comparator 11 and torch 3 is moved by a servo-motor 13 so that the difference thereof becomes zero.