PURPOSE: To make it possible to perform sealing of an opening of a multi-layered paper bag even when a peeling layer is not provided between an inner layer and an outer layer by bringing a double-folded overtape wherein a hot-melt adhesive is laminated on an inner face of a composite paper into contact with an outer layer of the outside of a welded part, and sealing the opening of a heavy-duty packaging paper bag by heat sealing at a temp. of a m.p. or lower of the inner layer.

CONSTITUTION: A multi-layered paper bag 11 is constituted of the innermost layer which is a synthetic resin inner layer 12, an intermediate layer 13 and an outer layer 14. An overtape 20 which is longer than the width of a multi-layered paper bag 11 forming a welded part 12A (the first heat welding) on the lower part of the inner layer 12 heated from the outside of the lower outer layer 14 of the multi-layered paper bag 11 by means of a heat seal machine is folded into a V-shape and is brought into contact with the outer surface of an opening 18 of the lower part of the multi- layered paper bag 11 and the overtape 20 and the outer layer 14 are adhered together by heating from the outside of the overtape 20 by means of the heat seal machine (the second heat welding).