PURPOSE: To launch simply, safely, and economically by providing an air cushion unit which shifts or launches a vessel by lifting a hull at a fixed platform or a sliding platform.

CONSTITUTION: A hull 1 is loaded on a slinding platform. On constructing a vessel, an air cushion system is not working, a sliding platform 2 being mounted stably on a ship way 5, which allows to support on the building berth 4 without providing a fixing method. After completing ship construction, the fan 8 of the air cushion system 6 is starting its operation, the air sucked into an air passage 7 from the side opening of the fixed platform 5 being boosted to compress the air cushion being developed within the air space surrounded with the top surface of the fixed platform 5, the bottom surface of the sliding platform 2 and the skirt 3, and permits to float the hull 1 by air pressure of the said cushion. Consequently, the friction developed between the hull 1 and the building berth 4 is remarkably reduced, and enables to shift simply the hull 1 with use of a small powered winch, etc.