To provide a device for preventing water area contamination diffusion of sink-and-float type capable of sinking the whole section requiring sinking without fail.

This device for preventing water area contamination diffusion of sink-and-float type includes a hollow cylindrical float 12, an anchor 15 for mooring the float 12, and an anchor rope 16 to sink the required section by letting air go out of the float 12. A hose 20 for discharging air passes through the hollow inside of the float 12 continuously and has flexibility and shape holding property for preventing the float from being crushed even after the air inside the float is discharged and the required section is sunk up to desired depth. Air ventilation is possible in the inside and the outside of the hose in the hollow inside of the float 12, and a tip of the hose for discharging air is opened into the outside of the float 12 to form an air supply and exhaust opening 21.