PURPOSE: To prevent the generation of an error in copying a weld line without requiring a large-scale device by determining the magnitude in the luminance just before the molten pool and making the min. value of reflected light and the deviation of an arc center to coincide by a servocontrol mechanism.

CONSTITUTION: The strong light from arc light and scattering light of a short wavelength are eliminated by a band-pass filter 2 provided on the front face of a television camera 1, and the tip of a molten metal 9 is visualized. The video picked up by the camera 1 is scanned and luminance is measured. Thereis the part where the luminance is min. in the place where there is no molten metal 9 as there is a dark point in the root part 10 of a weld groove. If such min. point exits on the same line on ≥2 scanning lines, said point is decided as the part 10. The distance between the video center and the part 10, and aservocontrol mechanism 12 is moved by said signal, whereby the welding center and the welding groove are matched.