To eliminate the difficulty of rotating a dual door system following connection between a cover and a door body and to eliminate wear or the like of a sealing member during this time.

A cover and a door body are divided into inner parts 12 and 38 and outer parts 13 and 39. Conical surfaces 18 and 42 are provided in the inner parts, sealing is secured for a shoulder part by expanding a sealing member in a radial direction and sudden shifting of the cover or the door body is prevented. The outer parts are opened by an air system, the assembly of the cover and the door body is removed and a drum and a container inside are connected to each other. Since it is not necessary to rotate the drum for locking the drum and a wall body at all, no friction occurs.