PURPOSE: To always accurately profile the center of groove width by correcting the output of a deflection detection magnetic sensor by the output of a step quantity detection magnetic sensor.

CONSTITUTION: The magnetic sensor 16 to detect deflection of a welding torch with respect to the central position in the direction of groove width of a butt joint and the magnetic sensor 1 to detect the step quantity of the butt joint are arranged. The welding torch is moved along the groove center according to the output of the deflection detection magnetic sensor 16 and the welding torch is held at the prescribed height according to the output of the step quantity detection magnetic sensor 17. When the butt joint has a step, the output of the deflection detection magnetic sensor 16 is corrected by the output of the step quantity detection magnetic sensor 17. By this method, accurate profiling of a weld line with respect to a square groove butt joint can be performed under noncontact state and the center of groove width can be always accurately profiled.