Provided is a plug for preventing leakage of carbonic acid gas in a PET bottle or glass bottle containing soft drinks to maintain freshness and original taste of soft drinks for an extended period of time. The plug for preventing leakage of carbonic acid gas has a small cork(300) that is formed in a cylindrical shape of a designated length. The small cork(300) and the plug(203) together serve to prevent leakage of carbonic acid gas from a soft drink bottle. The small cork(300) includes a weight and a groove(800). When a user drinks, the weight moves towards the opening and the cork(300) moves downwardly from the plug(203). The room created after some of the drink is escaped is filled with air having pushed carbonic acid inside the bottle.
