To make an inner stopper unremovable by contriving so that nails, tools or the like is hardly available for catching on at the lower end of a skirt part by a method wherein the skirt part for fitting to a mouth of a container is formed at the outer circumference of the inner stopper, and a thin part is formed at the edge of the skirt part so that the thin part comes into contact with the outer circumference at a neck of the container.

An inner stopper 4 is formed of synthetic resin having a moderate flexibility, and its thin part 6 deforms when being attached to a mouth 2 and pressed in so as to be fit to a stepped part. Therefore, the thin part 6 is supported in the direction of radius at the stepped part 2b and becomes inflexible with an intensified strength against a force applied from the outside provided. With the stepped part 2b provided at a position right behind the thin part 6, an edged tool, even though being inserted between the inner stopper 4 and a container 1, can be inserted barely into a small depth. Therefore, an area for pushing up the inner stopper 4 by the edged tool becomes small, and a force sufficiently strong for removing the inner stopper 4 cannot be applied thereto.