PURPOSE: To prevent the generation of surface vortexes by means of a simple constitution and provide an air self-supply type agitator floating on the water level by providing a float plate floating and covering the water level area and an air suction flow path with its one end opening above the water level and the other end opening on the rear face of an agitating element.

CONSTITUTION: When an air self-supply agitator 11 is floated on the water level of an area to be aerated by the buoyancy of a float plate 13 and the agitator 12 is operated, fluid on the rear side is sucked and jetted on the front face side by the rotation of an agitating element 12a to generate an agitating flow. Water masses remaining on the surface layer of the area to be aerated are agitated, and the water masses on the surface layer are transferred to a deep layer by the agitating flow, and water masses in the deep layer are agitated and mixed with the water masses in the surface layer. At that time, an upper water level area of the agitator 12 is covered by the float plate 13 to prevent the generation of surface vortexes. On the other hand, the rear face side of the agitating element 12a is in the negative pressure state when the agitator 12 is operated, and air is sucked from an opening 14a through a pipe 14 and diffused from an opening 14b into the agitating flow and aerated.