PURPOSE: To perform the easily-controllable water treatment of high efficiency with simplified arrangements, by sucking air using potential energy, and forming turbulence to wash a biomedium and to hold biomembranes under a fresh condition.

CONSTITUTION: An exhaust pipe 6 equipped with a check valve 7 is provided at the upper part of a closed catalytic oxidation tank 1, while a pipe 8 for supplying or draining water is provided at the lower part. The other end of said pipe 8 is connected to the spout of a lift pump 10, and an air-sucking pipe 4 through which raw water does not flow out is provided at a position above the pipe 8. The supply of electric power to the pump 10 and its disconnection are controlled by an electric controller 9. Hence, air is effectively sucked in by energy derived from the height, and a biomedium is effectively washed by a violent turbulent flow formed at the same time. Thus, the treated water of high purity is obtained with high efficiency.