PURPOSE: To rapidly deal with the inclination of a hull due to the variation of hull load, by generation adding drainage buoyancy by filling a gaseous body into air bags, which are mounted at the appropriate locations of an underwater portion or a lower hull of a half-underwater ship, and by discharging the gaseous body.

CONSTITUTION: A proper number of drainage buoyancy generating device 16 containing air bags 20 are installed at the pertinent locations of an underwater portion of a half-underwater ship - that is, support portion 4 under a horizontal plane A. Said gas bags 20 are supplied with pressurized gas from supply pipes 24 and expanded to the locations of two-dot chain lines, and drainage buoyancy corresponding to the volume of the gas bags 20 is produced, thus quickly coping with the sudden tilt of a hull due to the fluctuation of hull load. Meanwhile, gas in said gas bags 20 is discharged through discharge pipes 25, and the air bags 20 contract.