PURPOSE: To provide parallel seam welding for sealing a case of relatively large dimensions with a relatively small power source by providing a good conductor layer on or contacting a good conductive surface with the lid of the case.

CONSTITUTION: A surface of a lid 4 excluding areas in contact with welding electrodes 5, 6 is covered and contacted with a gold or other metal plated surface or a pasty conductor layer. Alomst no electric energy is consumed at the thus covered areas which situate very low, but mostly at the areas where the lid is contacted with the welding electrodes. Thus, without consumption of excessive energy, but with a relatively small source of energy, sealing by means of parallel seam welding techniques can be applied to a case of relatively large dimensions.