PURPOSE: To protect a hull and improve safety against the roll of the hull by allowing an air bag to bulge instantaneously to the outer face from the whole periphery of a vessel's space approximately along the draught line of the hull when occasion demands so as to form this air bag into a buffer body at the time of collision or the like.

CONSTITUTION: The interior of a hull, provided with a single swing closable hatch 2 pivotally fixed to a hull frame approximately along the draught line across the whole periphery of a vessel's space in such a way as to be opened downward into the almost horizontal position around the lower side part, is isolated in the water-tight state to provide an air bag storage chamber 3. An air bag 5, formed of deflectable material, with its interior divided into plural chambers by partition walls, is enclosed in the folded state inside the air bag storage chamber 3, and compressed air lead-in ports provided chamber by chamber are respectively communicated with a compressed air cylinder. At the time of sensing the danger of an accident such as collision being generated, the closable hatch 2 is opened, and simultaneously the valve of the compressed air cylinder is opened to supply compressed air into the air bag 5. The air bag 5 is thereby expanded in an instant to form an air cushion buffer body.