PURPOSE: To set a welding torch to a weld zone always at high accuracy and perform automatic copying welding, by detecting positional deviation in two dimensional directions between a welding torch that moves in a body with an image pickup device and a weld zone.

CONSTITUTION: In this automatic copying welding method, a slit light 12 of, for instance, laser is projected from an illuminator 11 to a weld zone 18 of horizontal fillet welding consisting of two butted base metals 10a, 10b, and a slit image 12' is formed. After taking the image 12' by an image pickup device 13 as slit image data, it is picture processed in a signal processing section 14, and reference data and positional deviation data are obtained. Amount of deviation is detected from both data, and the amount is supplied to a welding torch driving device 15, and a welding torch 16 that moves in a body with the device 13 is set to the weld zone 18. The device 13 and illuminator 11 are so set that their optical axes cross at 90°.