To reduce oscillation of a main floating body structure loaded with a hotel or the like or provided with an aircraft runway in a large floating body structure.

A large floating body structure is provided with a peripheral part floating structure 3 disposed with a space to a main floating body structure 5 so as to surround the periphery of the main floating body structure 5, and a fender 4 is interposed between the peripheral part floating body structure 3 and the main floating body structure 5. Since draft of the peripheral part floating body structure 3 is set larger than draft of the main floating body structure 5, even in case the peripheral part floating body structure 3 is oscillated in large degree by waves, oscillation of the main floating body structure 5 loaded with a hotel or the like is reduced.