To automatically remove sand deposited in the vicinity of a river- mouth by storing water in a gate arranged so as to be openable/closable, and discharging the water in the gate when a water level outside of the gate lowers more than the inside of the gate.

At tide level rising time, water by a flow of a river is stored on the inside (the upriver side) of a gate 1. At high water time, when a height of the gate 1 is not more than high water time, the water flows inside the gate 1 by overflowing, and when the height of the gate is higher than high water time, the water is stopped by the gate 1. At this time, sand is deposited on the river-mouth side from the gate 1 by a tide. At tide level lowering time, water by the water by a flow of the river and overflowing seawater is stored inside the gate 1, so that a fairly water level is maintained. Since the gate 1 is opened when a water level outside of the gate 1 lowers more than the inside of the gate 1, the water stored inside the gate 1 is discharged at a stretch, so that the sand deposited on the river-mouth side is flowed/removed by the water flow.