A plug structure is provided that can be installed on a container in which the thread of a container inlet for installing a cap is an outer thread, and furthermore, bubbles are not produced at the fluid surface inside the container even during fluid circulation. In a plug structure that is used in a container in which the thread of the container inlet for attaching the cap is an external thread, that is installed on the container inlet, that is capable of removing a fluid inside the container by using the siphon hose method, and that is capable of circulating the fluid that is inside the container, a fluid circulation path is provided that causes a fluid that is returning into the container during fluid circulation to fall along an external wall of a siphon hose during fluid removal.