PURPOSE: To manufacture a square bag in a series of operations ranging from the feeding of raw material to the formation of product by a method wherein thermoplastic synthetic resin foamed materials in continuous lengths are made in two sheets so as to tuck both ends of them in V-shape in order to cut off and weld them in the direction normal to the longitudinal direction and, after that, cut off and weld the tucked sides in the oblique directions.

CONSTITUTION: Synthetic resin foamed sheets S are drawn out of two raw sheet feed rolls 2 by means of first feed rollers 3. Both ends (t) of the sheets are respectively welded each other with a side sealer 4. V-shaped tucked parts d' are formed on the synthetic resin foamed sheets with supporting members 6 by pressing the ends (t) inwards with a disc-shaped tucking member 5. The sheets S are fed with second feed rollers 8 to dancer rollers 7 so as to remove the looseness of the synthetic resin foamed sheets S. The resultant sheets S are welded and cut off with impulse heaters 10 and shear cutters 11. Further, each tucked part d' is welded and cut off obliquely or at a bevel of almost 45° with an inverted-V shaped sealing cutter 12 so as to put the intersecting point of the welded bottom (a) and the depth (t) of the tucked part d' at the projected end of the inverted-V. Finally, the synthetic resin foamed sheets S are shifted to a product pan 14 by means of a carriage conveyer 13.