PURPOSE: To prevent the deterioration of the precision to detect the groove position by the fluctuation of the distance of an eddy current type displacement sensor to a base metal by fixing the center position of a welding torch at the middle point In the Z-direction between a first eddy current type displacement sensor and a third eddy current type displacement sensor.

CONSTITUTION: A set of first and second sensors are simultaneously driven in the groove width direction (z) so that the difference signal of the detected signals from first and second eddy current type displacement sensors 23a, 23b agrees with the difference signal of the detected signals from third and fourth eddy current type displacement sensors 24a, 24b. The first, second, third and fourth sensors 23a, 23b, 24a, 24b are driven so that the distance from the upper surface of a first member 1a to be welded to the oblique side of the groove immediately below the first sensor 23a agrees with the distance from the upper surface of a second member 1b to be welded to the oblique side of the groove immediately below the third sensor 24a. The first to fourth sensors are positioned where the groove center line is at the middle point between the first and third sensor 23a, 24a. Thus, the welding center of the welding torch is positioned at the middle point in the (z) direction of the first sensor 23a and the third sensor 24a.