When a three-dimensional structure is formed by welding a plurality of members, accurate welding can be performed matching the three-dimensional shape. The teaching data consisting of the three-dimensional position at each of the welding points determined on each of the welding lines at a predetermined spacing on the basis of the design data of the three-dimensional structure is stored in a memory 18. A laser slit light sensor (a laser slit light emitting section 12 and a CD camera 13) is controlled by a total controller 17 via a camera controller 14 to photograph the welding point in order to obtain the image data of the welding point. An image processing apparatus 16 processes the image data to determine the amount of deviation from the design data of welding points. The total controller modify the teaching data on the basis of the amount of deviation from the design data of welding points to establish the modified teaching data and controls a welding head so that it moves according to the modified teaching data.