To enhance the mounting strength of a float connecting implement in a pollution diffusion preventing device.

The pollution diffusion preventing device constituted to connect unit bodies each formed of a float and a curtain at least by a connecting implement mounted to the end of a tension belt provided at the lower edge of the float, adopts a means for forming the end of the tension belt as a multilayered part, interposing reinforcing plates at the multilayered part, clamping both front and rear sides of the center material formed of the float and curtain, the multilayered part and the reinforcing plates by connecting implement bases with D-rings mounted to the ends, boring a plurality of through holes serving for the communication of the connecting implement bases, the multilayered part, the reinforcing plates, and a center material, and tightening the respective through holes by suitable fasteners to fix them. The multilayered part is formed by folding first tension belts to hold the reinforcing plates therein, and further overlapping second tension belts on the outside.