To make it possible to raise a door body even if it is damaged by an impact of vehicle traffic or falling objects when it is in a lowered state, and to keep the door body in a raised state even if it is damaged by a water pressure of a tsunami or a high tide when it is in a raised state. A door body 2 of a floating flap gate 1 is disposed at an opening or at an access way. When water flows in, a forward end 2b of which swings upwards, in a direction in which the water flows in and within a plane in a height direction, around a base end 2a thereof which serves as a center of rotation, to block the opening or the access way. The door body 2 is formed from a hard polyurethane foam 4 in the form of a sheet.