PURPOSE: To facilitate manufacture and repairing by overhaul or replacement of constituting members by a method wherein a core frame is inserted into an insertion part between an outer member and an inner cloth of a gusset, and the outer member and inner cloth are seamed closely on the periphery.

CONSTITUTION: In a bag to open a lid both forward and rearward of a gusset, a gusset inner cloth 5 is arranged inside of a gusset outer member 4, an inner partition 6 is sewed to be attached to the gusset inner cloth 5, an insert part 8 which opens or closes either front or back of the gusset part is formed be tween the gusset outer member 4 and gusset inner cloth 5. A core frame 11 which keeps the shape of the gusset is inserted into the insertion part 10, and then the open-close sides of the gusset inner cloth 6 and gusset outer member 4 are seamed. By this constitution, a bag can be manufactured without sewing the core frame 9 of hard material with the inner partition 6 or gusset inner cloth 5. Further, as the core frame 9 is not sewed any other member, the core frame 9 can easily be taken out for overhaul for repairing.