To surely prevent the air in a work chamber from leaking to the outside of a caisson body automatically without a need for human labor.

The air leakage prevention device 10 in a pneumatic caisson method includes an underground water pressure detecting means (water pressure metering sensors 28a-28d) detecting underground water pressure Pw of the ground G near a cutting edge of a blade 14 of a caisson body 11, an in-caisson actual air pressure detecting means (an air pressure metering sensor 30) detecting an in-caisson actual air pressure Pc inside the work chamber 16, and a pressure control means (a controller 35). The pressure control means sets a standard in-caisson air pressure P1 in the work chamber 16 for obtaining the water level inside the work chamber that can prevent the air from leaking to the outside of the blade 14 based on the underground water pressure Pw detected by the underground water pressure detecting means (28a-28d), and adjusts the air pressure inside the work chamber 16 so that the in-caisson actual air pressure Pc can come closer to the standard in-caisson air pressure P1.