To stabilize an internal floating body structure by forming a water surface within a cylindrical part extending downward by being connected to the peripheral edge of a deck opening formed on an upper deck and gently restricting and mooring the internal floating body structure floating on the same water surface and the outer peripheral surface in the inner peripheral part of a cylindrical body.

A cylindrical part 5a of a square cylinder extending downward by being connected to the peripheral edge of a rectangular deck opening 5 at a required location of an upper deck 3a of a large-sized floating body 3 provided with the huge upper deck 3a provided for a maritime heliport, etc., is provided. A building like a flight control tower is floated as an internal floating body structure 4 on an internal water surface 7. The outer peripheral surface of the internal floating body structure 4 is gently restricted in the inner peripheral part of the cylindrical part 5a via a plurality of fenders 6. As a result, the internal floating body structure 4 floating on the internal water surface 7 which is calmly kept becomes an unfluctuating state even if waves are generated on the external water surface and the large-sized floating body 3 fluctuates.