To shorten a use period of a dock.

In this buoyancy device for a vessel structure, a first and a second buoyancy blocks 11, 12 installed along width of a vessel structure on curved surfaces of a vessel bottom on the bow side and the stern side of a PE block 13, a sensor 20 to detect axial inclination and lateral inclination of the block 13, and a controller 21 to connect the first and the second buoyancy blocks 11, 12 to the sensor 20 are provided, the buoyancy block 11, 12 is composed of plural boxes 121-1212 connected to each other by pipes except for those adjacent to each other at the lowest part of the vessel bottom of the vessel structure, intake valves 17, 18 for taking in brine are provided on the box 12 positioned at the uppermost part of the vessel bottom of the vessel structure, and extracting valves 15, 16 for extracting brine after construction are provided on the box 12 positioned at the lowest part of the vessel bottom of the vessel structure.