a heat sealing device in a packaging apparatus utilizing a plastic film for continuously seal an overlapped axial portion of the plastic film . the device comprises a pair of heating rollers which are rotatably and oppositely placed to hold the overlapped portion of the film between them . each of the heating rollers has a heating surface in its outer periphery which is pressed against the film and a coaxial and annular sealed chamber which is formed inside the heating surface and contains working liquid as a heat medium . a heat source is provided to be opposite at least to a portion of the sealed chamber to heat the working liquid . the working liquid generates vaporized gas upon heating by the heat source , and the pressure and the temperature in the sealed chamber are maintained at a predetermined degree so that the vaporized gas is condensed substantially at the temperature for fusing the film .