An automatic seam welding apparatus for fuel tanks comprises a jig for holding two workpieces disposed one above the other to form a fuel tank, a pair of roller electrodes vertically arranged for nipping and welding the overlapping peripheral flange portion of the workpieces, and a robot assembly rotatably supporting the jig and controlling the horizontal and vertical positions and the inclination of the jig held workpieces relative to the electrodes for welding peripheral flange portions that are irregular with high quality welds. The jig is easily detachable from the robot to permit many different shaped tanks to be welded by the same apparatus.