To facilitate drainage at normal time while prevent backflow of drainage automatically if required by opening a door through application of rotational moment around a horizontal support to the door by using a floating force generating at a floating part which is formed by closing an opening of the door with a plate according to a rise in level of water in a drainage receiver.

A gutter pipe 11 serving as a drainage pipe is layed to penetrate a bank so as to perform drainage to a river 12 being a drainage receiver from a land and a gate 13 is arranged at an end thereof. The gate 13 is constituted of a door abutting part 15 fixed to a drainage opening 14 at a tip of the gutter pipe 11, a holder part 16, and a door 18 supported so as to be swingable through a horizontal support shaft 17. When the level of water in a river 12 rises due to an increase of water therein, a floating force F is generated in a floating part 22 formed by closing a recessed part 20 opened in the door with a plate 21, and it is applied to the center of volume V of the door 18 to thereby apply a moment around the shaft 17 to close the door 18 thereto, resulting in bringing the door 18 into close contact with the part 15.