The invention relates to a closure device (2) for closing and/or opening an opening (1.1) of a container (1), wherein the opening (1.1) of the container (1) is closed in a first position (I) of the closure device (2) and the opening (1.1) of the container (1) is open in a second position (II) of the closure device (2), and wherein the opening (1.1) of the container (1) has an opening periphery (1.2) which comprises an inner wall (1.3) and outer wall (1.4), and wherein the closure device (2) has a sealing element (4) by means of which the opening (1.1) of the container (1) can be closed with sealing action in the first position (I), and wherein the closure device (2), in the first position (I), is fastened on the container (1) by a closure element (3) and, when the container (1) is opened, can be transferred from the first position (I) into the second position (II) by means of at least one actuating element (3.2) wherein the container opening (1.1) can be opened only by pressure (Pt) being applied to the actuating element (3.2).