To resolve some problems found in the prior art cap for a PET bottle that user's both hands are required for temporarily opening or closing a lid due to its screw-type or sporting-cap type and that requires an expensive manufacturing cost.

An artificial cork is attached to either the upper surface or the lower surface of a bottle cap so as to form an inner plug. A shape of the artificial cork is set to have such a shape as one in which it has a contact area and a height enabling the plug to be sealingly closed under a frictional force not easily opening it in such a way that the plug can be applied repetitively with the artificial cork without modifying a body of an existing PET bottle or a screw structure inside the cap. In view of its sanitary state, it does not generate any resistance in feeling against its use like a pull-tab open type beverage can.