A welded structure is formed by arranging multiple pieces on a three-dimensional support base to provide an assembly in accordance with the structure to be welded and making a picture map of the assembly by photographing the assembly. Weld points are identified from the picture map and welding parameters are determined from the picture map. Control data specifying the weld points and welding parameters are supplied to welding equipment that is operable in a three-dimensional X-Y-Z coordinate system. The welding equipment is moved in the X-Y plane on the basis of the picture map and said control data. The welding equipment is moved along the Z-axis on the basis of Z-axis position data that are acquired by repeatedly measuring the Z-axis position of the support base.