The present invention provides an inner plug easily removable from a neck of a container, and a cap for covering the inner plug. The inner plug comprises an engaging portion to be fitted to a neck of the container, a threaded cylinder, and a lip cylinder connected to the threaded cylinder. The engaging portion comprises an inner cylinder, an outer cylinder, and a shoulder wall. The threaded cylinder is arranged on the inner cylinder. The cap has a side cylinder to be engaged with the threaded cylinder of the inner plug, and a cover cylinder covering the shoulder wall and the outer cylinder. The cover cylinder has a diameter larger than a diameter of the side cylinder. The outer cylinder is provided with a cut portion. The shoulder wall is provided with a cut portion connecting to the cut portion of the outer cylinder. The shoulder wall is provided on a lower surface thereof with a tear off groove along the outer cylinder. The groove is connected to the cut portion of the shoulder wall. Also, a jig for holding an inner plug unrotatably is provided.