Abstract: A device for the strip-shaped heat sealing or welding of the inner walls composed of a thermoplastic material and lying on top of one another of tube sections that are lying flat is comprised of two heated, longitudinally extending heating jaws, which limit a gap between one another, of means for pressing the tube sections together in one region of the heating jaws, and of a conveyor device that moves the tube sections through the gap. In order to prevent the tube sections from becoming offset relative to their conveyor devices, these means comprise a pair of rolls mounted in the region of the heating jaws, whose rolls form a pressing throughput nip for each tube section.