To properly reduce an internal pressure of an air bag type stern seal without causing any loss of high pressure air so as to ease interference with waves, especially when a ship navigates on a rough sea, in an air cushion ship.

High pressure air fed to an air bag type stern seal 3 of an air cushion ship by means of an air supplying fan 5 is distributed to an air cushion chamber 4 via an air extraction port, 14a and an air distributing inlet 11a, so that an internal pressure in the stern seal 3 is regulated by regulating an opening of a louver 11 in the air distributing inlet 11a. When the ship navigates on a rough sea, an internal pressure in the stern seal 3 is reduced and interference of the stem seal 3 with the waves can be eased.