PURPOSE: To facilitate the uniform and sure fixing of materials to be welded by disposing pads on the clamping surfaces of clamping parts and inserting elastic bodies between the pads and the clamping surfaces.

CONSTITUTION: The pads 5 are mounted to the clamping surfaces of the clamping bars 5', 5" of a clamping device 5 and the rubber plates 8 are inserted between these pads. The material 1 to be welded is clamped by the clamping bars 5', 5". Sufficient pressing force is applied to the material 1 to be welded even if the parallelism and flatness of the respective parts are not so much strictly accurate. The material 2 to be welded is clamped by the clamping device 6 made into the same construction and is superposed on the material 1 to be welded. Electrodes 3, 4 roll while applying the pressing force to these materials and currents are passed to the electrodes to weld the materials 1, 2 to be welded. The materials 1, 2 to be welded are evenly and surely fixed over the entire direction in the transverse direction thereof and the relief at the time of welding is minimized.