An apparatus for making flat packaging bags from a flexible web of indeterminate length includes a machine frame; a sled mounted on the machine frame for travel parallel to the direction of web advance; a first drive for reciprocating the sled parallel to the direction of advance; a mandrel mounted on the sled; a tubular folding body fixedly mounted on the machine frame and coaxially surrounding the mandrel for bending the web about the mandrel to form a web hose; a first sealing shoe mounted on the sled; a second drive for pressing the first sealing shoe against an outer face of the mandrel for providing a longitudinal sealing seam on the web hose situated between the mandrel and the first sealing shoe; a second sealing shoe mounted on the machine frame for displacements perpendicularly to the direction of web advance; a third drive for actuating the second sealing shoe for providing a transverse sealing seam on the web hose; and a cutter for severing a length of web hose from the web.