PURPOSE: To eliminate the need for teaching the sensing direction of a Y-shaped stylus and to simplify an operation by attaching said stylus to the side face of a welding torch, obtaining the vector in the direction of the welding torch and determining the sensing direction of the stylus.

CONSTITUTION: A Y-shaped stylus is used. The tip of a welding wire W is matched with the corner part and the stylus is attached to the side face of a welding torch T in such a way that the axial direction thereof is made parallel with the torch and that the stylus is made freely extensible and contractable by means of a cylinder 3. The torch T is moved in the respective directions of the stylus and the positional deviation of a member P is detected by the short-circuit between side torch and the case attached to the number P. The vector in the direction of the torch T attached with stylus sensors 1, 2 is obtd. and the sensing direction for moving the sensors in the direction of the member to be welded is determined in the ±45° direction. Welding is accomplished while the welding is procressed while reversing from the final sensing point toward the first sensing point.