| CPC E02B 7/44 (2013.01) | 8 Claims |

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1. A watertight structure for a flap gate including a door, the door being supported in a weight balance in which the door opens an opening portion of a floodgate in a state in which no external force other than gravity is applied, the watertight structure comprising:
a gap opening-closing member that provides a gap between a lower edge of the door and a bottom face of the floodgate in a state in which the door closes the floodgate, and opens and closes the gap by swinging in a water flow direction;
opening operation assist means for operating the gap opening-closing member into an open state by lifting up the gap opening-closing member to an outer water side; and
closing operation assist means for operating the gap opening-closing member held in the open state into a closed state by buoyancy generated when submerged in water.
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