To dispense with a large scaled breakwater to sufficiently control oscillation due to wave by severally setting the numerical aperture of a perforated plate less than a specific percent, a length along the wave progress direction of the perforated plate specified times as long as the objective wave length and the depth from the surface of water of the perforated plate less than specific times of the objective wave amplitude.

A floating structure 4 itself is provided with a wave extinguishing structure which is composed of a stretching plate 13 which extends horizontally above the surface of the water from a floating structure at the end on the wave receiving side of a floating structure 1; many cylindrical bodies 12 which pierce the surface of the water 21 from the plate 13 and are vertically provided; and a perforated plate 1 which is joined to the lower end of the body 12 and extends horizontally. In addition, many circular openings 14 are formed on the plate 1. The numerical aperture of, the plate 1 is set less than 50%, and the length L along a wave progress direction of the plate 1 is set 0.6-1.2 times as long as the objective wave length of wave, and the depth (d) from the surface of the water of the plate 1 is set less than 1.5 times as wide as the objective wave amplitude hA of the wave. The oscillation of the floating structure 4 can be controlled.