PURPOSE: To continuously manufacture a commodity packing bag having a gusset structure at side edges thereof, by providing a thick sheet with gussets of the same width and, melting and cutting an upper edge of the wrapping bag and cutting a lower edge mechanically at the cutting process of the long laminate with the gusset.

CONSTITUTION: In process A, a bottom film 2 and an upper film 1 are folded and overlapped at a center line 5, and the obtained film is taken out to process B. Then, the long laminate is sent to process C, where heat seals 4, 4' are thermally melted and fixed to both edges of a thick sheet 3. In the next process D, the thick sheet is melted and attached to the bottom film 2, thereby to form a long laminate of a D cross section. The laminate is carried into an E process. Gussets 7, 8 are provided so that both edges 5, 6 of the tubular thin film D are positioned over the thick sheet 3, resulting in an E cross section. The long sheet of the E cross section is fed to a cutting process for cutting.