To enable micro-animals-fixed carriers to be mixed in gentle circulating condition and uniformly by forming uniformly circulating current by rotation of rotary blades within a vertical plane provided inside a cylindrical reaction tank and made up of a water permeable material.

The reactor is provided with rotary blades 11 made up of a water-permeable material within a reaction tank 4 packed with carriers. A raw water inlet 1 is provided on the upper part of the reaction tank 4 and raw water to be treated flows in therefrom. The reaction tank 4 is a cylindrical form installed sideways to the vertical direction and inside provided with rotary blades 11 provided on a shaft concentric with the center shaft of the reaction tank 4. The rotary blades 11 are rotated by current produced by rise of bubbles through ventilation from an air diffusion plate 12 provided at a lower part. The circulating current circulating within the reactor tank 4 is promoted by the rotation of the rotary blades 11. The rotating direction of the rotary blades 11 is counterclockwise in a view that a treated water outlet 2 is on the right and the air diffusion plate 12 is below.