PURPOSE: To obtain a bag fitted with a fastener having a high sealing degree not simply opening a fastener even when the bag is filled with content and pressure is applied to the bag from the inside.

CONSTITUTION: In a bag fitted with a fastener, one of the films joined to the meshing part of femal and male fastener pawls is folded back and an opening part is set to an overlap system. This bag is produced by folding back a film fitted with a fastener having one of string fastener consisting of two narrow films respectively having the female and male fasteners 2, 3 welded thereto downwardly so that the meshing part of the fastener becomes the vicinity of the end part of the bag and cutting the same to form a bag.