PURPOSE: To carry out the welding of foil without deformation by bending the butt parts facing each other of the foil in parallel with a weld line, then butting and welding the bent parts against and to each other.

CONSTITUTION: Materials 1 to be joined which are sheets, i.e., foil, of ≤0.3mm thickness are butted and welded against and to each other. The butt parts 2 facing each other of the materials (foil) 1 to be welded are respectively bent along the weld line in parallel therewith to form the bent parts 3. The bent parts 3 are butted and welded 6 against and to each other. The weld zones of the foil in the case of superposing and welding of ≥2 sheets of the foil are bent respective in parallel with the weld zone to form the projecting parts and the respective projecting parts are lap welded. The generation of such weld defects as melt down and holding of plates, is obviated in this way.