To increase the bending strength to be able to stand the force of waves by reducing the height of a main floating structure and simplifying the interior structure.

A plurality of reinforcement beams 10 that extend in the longitudinal direction of a main floating structure 8 are placed in parallel under the main pontoon-type floating structure 8. Supporting members 11 are attached on the top side of each reinforcement beam 10, and an exterior structure 9 is formed. The exterior structure 9 is attached so that its structure is integrated with under side of the main floating structure 8. Each reinforcement beam 10 is made so that its structure is integrated, connected by connecting members 12 that extend laterally. The support members 11 are also made so that they are trusses.