Abstract: A cap includes a metallic cap shell having a circular top and a skirt extending downwardly from the peripheral edge of the top and provided with a thread-forming portion deformable along the thread of the opening part of a container, and a flexible liner provided inwardly of the top of the cap shell. The liner includes an annular protrusion having an inside diameter substantially equal to, or slightly larger than, the outside diameter of the sealing surface of the opening of a container to be sealed and comprising a perpendicular inside wall adapted to seal intimately with the peripheral sealing surface of the container opening and an upright outside wall spaced apart from the inner circumferential surface of the skirt.