A closure system (3; 30) for closing an opening (25; 250) of a container (2; 20), comprising a stopper member made of an elastic material and having a plug portion configured to tightly fit into the opening (25; 250) of the container (2; 20) and a cover portion configured to abut a boundary surface (24; 240) adjacent to the opening (25; 250) of the container (2; 20), and a cage member (32; 320) made of a shape-retentive material, wherein the cage member (32; 320) has a stopper contacting section (321; 3210), a clipping structure (323; 3230) and in intermediate section (322; 3220) between the stopper contacting section (321; 3210) and the clipping structure (323; 3230), wherein the clipping structure (323; 3230) of the cage member (32; 320) is configured to clip in a corresponding structure (211; 210) of the container (2; 20), and wherein the cage member (32; 320) is configured such that the stopper contacting section (321; 3210) of the cage member (32; 320) applies a pressure to the cover portion of the stopper member to push the cover portion of the stopper member onto the boundary surface (24; 240) of the opening (25; 250) of the container (2; 20), when the plug portion of the stopper member is fitted into the opening (25; 250) of the container (2; 20) and the clipping structure (323; 3230) of the cage member (32; 320) clips in the corresponding structure (211; 210) of the container (2; 20), characterized in that the cover portion of the stopper member has a circumferential container notch and a circumferential cage notch, wherein the circumferential container notch of the cover portion of the stopper member is directed towards the boundary surface (24; 240) of the opening (25; 250) when the plug portion of the stopper member is fitted into the opening (25; 250) of the container (2; 20), wherein the circumferential cage notch of the cover portion of the stopper member is directed away from the boundary surface (24; 240) of the opening (25; 250) when the plug portion of the stopper member is fitted into the opening (25; 250) of the container (2; 20), and wherein the cage notch of the cover portion of the stopper member is radially offset relative to the container notch of the cover portion of the stopper member. a stopper member made of an elastic material and having a plug portion configured to tightly fit into the opening (25; 250) of the container (2; 20) and a cover portion configured to abut a boundary surface (24; 240) adjacent to the opening (25; 250) of the container (2; 20), and a cage member (32; 320) made of a shape-retentive material, wherein the cage member (32; 320) has a stopper contacting section (321; 3210), a clipping structure (323; 3230) and in intermediate section (322; 3220) between the stopper contacting section (321; 3210) and the clipping structure (323; 3230), wherein the clipping structure (323; 3230) of the cage member (32; 320) is configured to clip in a corresponding structure (211; 210) of the container (2; 20), and wherein the cage member (32; 320) is configured such that the stopper contacting section (321; 3210) of the cage member (32; 320) applies a pressure to the cover portion of the stopper member to push the cover portion of the stopper member onto the boundary surface (24; 240) of the opening (25; 250) of the container (2; 20), when the plug portion of the stopper member is fitted into the opening (25; 250) of the container (2; 20) and the clipping structure (323; 3230) of the cage member (32; 320) clips in the corresponding structure (211; 210) of the container (2; 20), characterized in that the cover portion of the stopper member has a circumferential container notch and a circumferential cage notch, wherein the circumferential container notch of the cover portion of the stopper member is directed towards the boundary surface (24; 240) of the opening (25; 250) when the plug portion of the stopper member is fitted into the opening (25; 250) of the container (2; 20), wherein the circumferential cage notch of the cover portion of the stopper member is directed away from the boundary surface (24; 240) of the opening (25; 250) when the plug portion of the stopper member is fitted into the opening (25; 250) of the container (2; 20), and wherein the cage notch of the cover portion of the stopper member is radially offset relative to the container notch of the cover portion of the stopper member.