PURPOSE: To detect a weld line with a small size and high sensitivity and to follow up the weld line by rotating reversibly a sensor which detects the distance up to an object to be welded around a specified revolving axis to detect a weld point and making the central axial line coincident with the weld line.

CONSTITUTION: A sensor 5 which detects and controls the height of a wire 4 by detecting the distance (d) up to an object 2 to be welded is reversibly rotated around the central line of the wire to detect the point on a weld line 2. A robot controller 13 is actuated by the directional angle θ thereof to advance an arm 14 in said direction thereby making the transfer of the central point coincident with the weld line. The height and the direction of the weld line 2 are detected by the distance sensor 5 and the weld line 2 is followed up by one small-sized height sensor.