To provide a highly calm sea area by surrounding a prescribed sea area by a floating body structure by excluding a ship entering/exiting opening part, arranging a half submerged floating body structure for integrally fixing a pier and a mooring pile in the sea area surrounded by the floating body structure, and constituting a floating body mariner by fixing a floating body bottom structure to the floating body structure.

A floating body structure 1 is built in a dock of a dockyard, is towed up to a prescribed sea area, is welded and joined on the sea of the actual site, and is formed as a structure for surrounding the prescribed sea area A to be moored to a mooring device 7. A floating body bottom structure 2 is formed as a divided body having a width dimension retractable from an opening part B in the prescribed sea area A surrounded by the floating body structure 1, and is built in the dock of the dockyard. A pier, a support pile and a mooring pile are also integrally built, are towed up to the actual site, and are successively pulled in the prescribed sea area A from the opening part B to be joined to the floating body structure 1 to thereby enhance calmness.