PURPOSE: To dissipate wave of wave length in wide range by arranging a vertical middle bent partition plate extending along both side relates between front and rear side plates of a lengthy pontoon type wave dissipation floating dike, properly setting width of free water surface, and providing water through holes on the bottom.

CONSTITUTION: Between front and rear parts side plates 2a, 2b of a pontoon 1, a vertical middle bent partition plate 4 formed out of an upper vertical plate 5, a lateral U-like plate 6, and a lower vertical plate 7 is arranged along both side plates, and vibrating water blocks 13, 14 having narrow and wide water surface width B1, B2 are formed. Water through holes 10, 11 are arranged on the contact parts with both vibrating water blocks 13, 14 of a bottom plate 3. The water surface width B1, B2 or length of the vibrating water blocks 13, 14 are properly set, and phase difference is generated between the sloshing or reciprocating natural vibration and surging of the pontoon 1 is generated to energy of dashed wave. Hereby, wave of wave length in wide range can be dissipated.