Abstract: Opening-closing device for a bag of flexible synthetic material obtained from a material in the form of a film, of the type comprising a threaded drum which is attached to a wall of the said bag and interacts with a stopper capable of being screwed onto the said drum and comprising teeth making it possible to cut out the wall in the region of the central orifice of the drum to permit the opening of the said bag, characterized in that it comprises a means which is integral with the drum (2) and/or the stopper (1) and makes it possible to protect the opposite wall of the bag against the teeth (5) of the said stopper.According to an alternative form, the shoulder (7) of the drum (2) comprises a substantially cylindrical collar (8) dimensioned in such a way that the ends of the teeth cannot exceed its edge (9).