A stopper for a food or beverage container in which a functional additive is contained. A stopper body is screw-engaged with a container, and includes an opening support at the lower end which indicates the open state of the stopper and an inward flange on the upper end. A pressure vessel includes an outward flange coupled with the stopper body and an open bottom portion. The pressure vessel contains the additive and nitrogen gas, and is compressed by an external force to discharge the additive. A blocking film is coupled to the lower end of the pressure vessel, and is torn by the external force such that the additive is inputted into the container. A protective cap is coupled to the stopper body to protect the pressure vessel, and is separable from the stopper body.