To efficiently generate a stream of water and diffuse fine bubbles of air in the horizontal direction in a water area where such treatment is required to purity the water area.

The device includes air passage members 13 which project radially from a propeller boss 2 of a screw propeller 1 so as to face a propeller vanes, and compressed air is supplied from an air compressor 10 to air blowoff parts 14 each provided with a porous plate at the rear edge of the member 13. Bubbles blowing off from the parts 14 are atomized into fine bubbles as the members 13 revolve and mixed into streams of water 9 from the screw propeller 1.