PURPOSE: To prevent the occurrence of the fluctuation of a door body with a float owing to waves by a method wherein when the door body with a float is brought into a fall, a lifting wire is lead out in a direction in which the door body with a float is caused to float.

CONSTITUTION: When a water level in a channel is low, a door body 1 with a float is caused to rise and fall. Simultaneously, rotation of a drum 9 in a wire rewinding direction is locked so that a wire 5 is prevented from being rewound in the direction of an arrow mark (e). In this case, since the wire 5 is not rewound, the door body 1 with a float can not float but is brought into a complete fall state and held in a rest state. Meanwhile, when a water level in the channel is high, the door body 1 with a float is caused to rise. Simultaneously, the drum 9 is locked so that the drum 9 is prevented from rotation by means of a force by which the door body 1 with a float is about to fall under an own weight. In this case, the door body with a float is incapable of rising and falling and can be held in a standing-upright state.