PURPOSE: To automatically supply and discharge water from each ballast tank, by operating water supplying and discharging pumps on the basis of each of values detected by seated condition detecors for detecting that passengers are seated and values detected by water level detectors for ballast tanks placed at side, front and rear parts of a hovercraft.

CONSTITUTION: In a passenger hovercraft, the detectors for detecting that the passengers are seated are provided in the interior of passenger seats, and the ballast tanks 6a, 6b, 7a, 7b placed at side, front and rear parts of the hovercraft are provided with detectors for the water levels in the tanks, respectively. On the basis of detection signals from the seated condition detectors and the ballast water level detectors, the supplying and discharging pumps 8 for supplying and discharging ballast water from the tanks are automatically started or stopped. Accordingly, appropriate quantities of ballast water can be automatically placed into each of the ballast tanks 6a, 6b, 7a, 7b in accordance with the number and locations of the passengers.