To secure strength and watertightness while preventing enlargement of a door body.

A tide gate for opening and closing an opening 101 of a building (structure) 100 includes an outer door 1 for covering the opening 101 and receiving a water pressure from the outer side, and an inner door 2 for closing the opening 101 on the inner side of the outer door 1 and securing the watertightness of a closed part. That is, since the outer door 1 is in charge of a function of receiving the water pressure and the inner door 2 is in charge of a function of the watertightness, the strength is maintained by the outer door 1 and the watertightness is maintained by the inner door 2. As a result, since the respective functions are shared and the watertightness can be maintained by the inner door 2 without configuring the outer door 1 to be more robust, the strength and the watertightness are secured while preventing the enlargement of the whole.