To provide a backflow-prevention gate which has the function of preventing backflow by automatically closing a water channel as the amount of water increases, allowing no water to leak when it is closed.

A backflow-prevention gate 10 has a door 12 supported on axis in such a way as to freely slant inside a water channel 11 and producing a floating force in water, a closure member 17 is attached to the outer periphery of the door 12, and door stop members 19, 20 that abut to the closure member 17 are provided on the side wall surface 15 and ceiling surface 18 of the water channel 11, respectively. Also, a flexible closure member 14 is placed at the part 12 of the door 12 supported on axis, and a closure member 16 which abuts to the flexible closure member 14 when the door 12 is held upright is installed at the door stop member 19.