FLAP GATE

A flap gate (1) includes a door body (2) and a flap ancillary part (3). When the door body (2) is in its down position, a movable end portion (24) of the door body (2) is located forward of a supported end portion (23). The door body (2) changes its position between the down position and a maximum up position by turning on the supported end portion (23) serving as a support. The flap ancillary part (3) applies tilt-up moment to the door body (2) only when the door body (2) is located in a position between the down position and a first position (i.e., position between the down position and the maximum up position). The flap ancillary part (3) also applies tilt-down moment to the door body (2) only when the door body (2) is located in a position between the maximum up position and a second position (i.e., position between the first position and the maximum up position). This simplifies the structure of the flap gate (1) that can speedily start to tilt up when water flows in and that can early start to tilt down when the water level has started to drop.