To enhance a gas dissolving speed and to transfer treated water to a next treatment process while continuously mixing gas with treated water.

Treated water is charged in a vessel 1 into which compressed air is introduced under pressure while centrifugal force is generated by rotating a rotary plate 4 to be allowed to continuously flow down toward the lower part of the vessel 1 while a water film is formed to the entire periphery of the inner wall of the vessel 1 to be brought into contact with compressed air. Air bubble-containing treated water stored in the lower part of the vessel can be transferred to a next treatment process continuously.