PURPOSE: To exactly operate a rising and falling gate by forming an air vent opening in the side of a gate whose lower end portion is rotatably set through hinges on the river bed.

CONSTITUTION: When the level of water in a water channel reaches a planned level, an automatic device is actuated to cause a gate 1 to fall and the gate 1 sinks into overflow water. In this case, air bubbles 13 formed inside the gate 1 are quickly discharged from an air vent opening 11. Damage to the gate 1 and the hinges 2 by buoyancy of the bubbles 13 can thus be prevented.