Method of detecting weld lines

In a method of detecting a weld line, the positions of the weld line and the welding wire are detected from the video signal of the weld, which is provided by an industrial television camera. The video signal of the weld is compared with a predetermined threshold so as to be binary-coded into a signal representing a dark part and a signal representing a bright part. Clock signals having a predetermined frequency are counted with the binary-coded signals as gating signals, thereby to count the lengths of the dark part and the bright parts. The count values are subjected to difference processing, so that the positions of the weld line and the welding wire are detected from the maximum values of the results of the difference processing.