Abstract: Disclosed herein is a device for sealing portions of an overlapped pair of single or composite films by contacting or facing an appropriately heated sealing surface of a heat sealing member. The heat sealing member is provided with the sealing surface in a part of its outer surface and with a sealed chamber therewithin in the vicinity of the sealing surface. The sealed chamber contains under decompression working liquid for evaporating and generating high-temperature condensable gas by being heated. The heat sealing member further has a heat source in the interior or the exterior thereof adjacent to at least a part of the sealed chamber for heating the working liquid.