Abstract: The device which forms the object of the present invention comprises flat element (1) more than half of which is subdivided longitudinally into two parts (2 and 3). The external faces of the element (1) are provided with longitudinal grooves (4) and the internal faces are provided with longitudinal parallel ribs (5). The element (1) is embraced by a bridge piece (6) with laterally projecting wings (7) and a number of inner ribs (8) which slide within the grooves (4). To seal the opening of the container the lips of the opening are simply fitted between the two parts (2 and 3). Once in this position, the bridge piece (6) is simply pushed along by pressing the projections (7) with the fingers such that as it advances, guided by the coupling which exists between the ribs (8) and the grooves (4), it applies pressure on the opening until it is perfectly sealed, the presence of the ribs (5) eliminating any tendency of the new seal to slip.