PURPOSE: To obtain an apparatus for forming a flat packing bag from a flexible film which can be highly efficiently and safely operated and can well function.

CONSTITUTION: A plurality of sliders 20 and 21 are supported on a trestle 10 so as to be freely movably each other in the longer direction and are driven by means of a crank driving 30. Each slider 20 and 21 supports a mandrel 25 to form each one film 2 surrounding the mandrel into a tube by means of each one folding body 13 fixed on the trestle. When the sliders 20 and 21 are moved in the front direction, each longer sealing grip 26 presses the mandrel 26 to form a longitudinal sealing seam. When it is reversely moved, the film tube is held by means of a transverse sealing grip 15 and a finished bag body 3 is cut by means of a knife 16. Therefore, this apparatus can be extremely efficiently, systematically and simply operated.