To provide a cap structure having an inside plug, which can be used without modifying a cap supply line in a filling process only by slightly increasing a length of an aluminum pilferproof(PP) cap.

The assembly structure comprises a glass bottle with a thread, the inside plug and the aluminum PP cap. The bottle includes a step provided peripherally from the inside of a mouth top. The inside plug includes an engaging portion comprising an inside plug upper portion and an inside plug lower portion which are provided across a cut piece to be engaged with the step of the mouth top of the bottle, which is provided inside a curved surface of the inside plug. The cut piece is cut when capping, whereby the inside plug upper portion and the lower portion are separated and thus a lower surface of the upper portion lowers until it comes into contact with the mouth top of the bottle. In the meanwhile, a thread groove is formed on the cap for fitting.