PURPOSE: To correct the deviation signal which is made inaccurate by noise, etc. by using a follow-up basic vector in the stage of starting welding and the follow-up basic vector to be updated at every several cycles of the succeeding weaving.

CONSTITUTION: The direction of a taught weld line 1 is regarded as a follow-up basic vector 4 in the stage of starting arc and the deviation of a welding torch 11 is corrected during the several cycles of weaving of the torch 11 from the start point 4As of said vector 4. The vector 4 is updated to a follow-up basic vector 4B having a start point 4Bs and an end point 4Be at the point of the time when said correction ends. The vector is thereafter updated to a follow-up basic vector 4C having a start point 4Cs and an end point 4Ce after several cycles of weaving in the same manner as with the vector 4B. The deviation indicating the deviation which is made inaccurate by noise, etc. is corrected by the above-mentioned method.