PURPOSE: To transport a marine structure to the site safely by towing without falling of the structure, by providing floats which can be flooded or discharged at right and left lower parts and by supporting a marine structure in a floater consisting of floats each of which is connected to each other via upper frame bodies.

CONSTITUTION: Floats 3 which can be flooded or discharged, are provided at right and left lower parts. Hollow columns 4 are erected at both fore and aft positions of these floats 3, and each column top is connected to each other with a truss structured frame body 5. The aft ends of floats 3 are also connected to each other by a truss structured frame body 6. The floats 3 are flooded or discharged to adjust their drafts. Then a marine structure such as a concrete caisson or platform is hung down through a supporting piece at the inside of right and left floats 3, and it is towed by sea to the site.