To easily open a gate body even in the case where a foreign substance is mixed or drain water is a little, and to close the gate body even in the case where a water level of a river is stagnant or the water level rises slowly.

In a flap gate 1, a balancing member 3 to which a suspension member 7 is connected is integrally fixed on an outer surface on a side opposed to a drain hole 21 in a gate body 2, and specific weights of the gate body 2 and the balancing member 3 are made different from each other. When the specific weight of the balancing member 3 is defined as a, its cubic volume is defined as c, the specific weight of the gate body 2 is defined as b and its cubic volume is defined as d, the specific weights and the cubic volumes are determined in such a manner that a predetermined relation is established.