PURPOSE: To achieve maintenance and repair in a dry condition in a set up place by reducing bending moment and shearing force.

CONSTITUTION: In a large scaled float structure, many surrounding walls 4 are provided in a lattice shape in a bottom surface 3 of the float structure 1, in the peripheral downward of which a wave dissipating plate 2 extended to sink in water is provided to reduce transmission of waves to the bottom surface 3, and compressed air 7, setting the pressure in accordance with weight of a building 5 loaded on, is accumulated in each surrounding wall to support the float structure itself floatd.