1. A methd of controlling a weld line tracing, comprising: composing a weld main path vector for relative movement of a welding torch along said weld line, a swing vector for swing movement of said welding torch relative to said weld line, and a correction vector formed by calculating the amount of correction of said correction vector based upon the deviation between a present reference amount and a detected amount of an electrical parameter of an arc of said welding torch and by determining the direction of said correction vector independently of information on the actual position of the welding torch on the basis of said swing vector, and controlling said welding torch based upon the composed vector to effect tracing of said welding torch along said weld line.