To effectively reduce vertical motion using a flanged protrusion in wavy seas as well as reduce resistance to tidal currents in calm seas, by fixing a foldable flanged protrusion to the outer cylindrical surface of a floating structure under the water.

A bag 4 that becomes a flanged protrusion by filling with sea water via a pump 6 for filling and discharging sea water is fixed to the cylindrical outer surface of a partially submerged floating structure such as a column 2 under the water, thus allowing vertical motion of the floating structure to be controlled. When the bag 4 is not in use, it bends downward and folds up by discharging sea water, thus reducing resistance to tidal currents.