To prevent a door body from shaking significantly when it rises. A floating body flap gate is provided which is disposed on a roadway surface in an opening; and causes a door body to rise to block the opening when water is trying to flow in from the opening by using a water pressure of the water which is trying to flow in and a buoyancy of the door body. The door body is formed with three door body blocks, which are separated in the vertical direction. These door body blocks which are separated in the vertical direction, are connected by rotation mechanisms for rotation at a specified angle within a vertical plane in a direction in which the water is trying to flow in from the opening. The door body is not subject to significant shaking, regardless of the water level at which the water tries to flow in from the opening.