To accelerate drainage even in small flow quantity by keeping a valve element open normally, and quickly easily close the valve element in abnormality without requiring human-made judgment and operation to surely prevent reverse flow.

In this flap valve, a pair of drain ports 15, 15 are formed in right and left of a lower part of a peripheral wall of a valve casing 14 connected to a gutter, and a pair of flap valve elements 16, 16 in right and left for opening and closing these drain ports 15, 15 are supported freely to be rocked around the axes of horizontal shafts 18, 18 over each drain port 15, 15. Furthermore, each flap valve element 16, 16 is provided with each floating force generating part 19, 19 for giving the closing directional moment to each flap valve element 16, 16 with a rise of water level of a river.