To provide a cut-off structure to be durable against repeated use and reliably prevent leakage of water to the interior of a caisson.

An air bag 2 is mounted on a bottom part 1a of a caisson 1. The air bag 2 is provided with a tube 4 formed of an elastically deformable material; a covering member 5 attached to a part of the outer surface of the tube 4; and an air valve 6 to feed air to the tube 4. The tube 4 is expanded by a pressed in air pressure. When the caisson 1 seats on a foundation part 3, the air bag 2 is adhered to the respective members between the bottom part 1a of the caisson 1 and the foundation part 3 to prevent leakage of water to the interior of the caisson 1.