To restrain a phenomenon of being blown when waves and a tide act on a membrane body by constituting the membrane body of a membrane member composed of rubber or a resin having a special specific gravity, and arranging a float in the upper end part of the membrane body.

Flanges 3 are projected on upper/lower/left/right both ends of respective semicylindrical bodies 2. An interposing material 4 is fixed to a float 1 by bolts 5 and nuts by using inserting holes of the flanges 3 and through holes of the interposing material 4 so that the lower end part and both side parts appear from the float 1 by interposing a rubber interposing material 4 between the plane parts of the semicylindrical bodies 2, 2. A membrane body 7 is suspended from the interposing material 4. This membrane body 7 is constituted of a membrane member 9 composed of plural rubbers or resin by embedding a reinforcing material 8 composed of a fiber material such as cloth, a wire net or an iron plate, and the specific gravity is set not less than 1.0. Therefore, a phenomenon of being blown is largely restrained even if waves and a tide act on the film body since weight of the whole film body 7 itself is secured.