NOVELTY - Wastewater treatment, involves oxidation treatment of supplying oxygen to wastewater in the presence of nitrifying bacteria and denitrifying bacterium, and converting ammoniacal nitrogen contained in wastewater into nitrite nitrogen using the nitrifying bacteria, and denitrifying process of converting nitrite nitrogen and ammoniacal nitrogen into nitrogen gas using the denitrifying bacterium.    USE - Wastewater treatment method.    ADVANTAGE - The dissolved oxygen concentration of wastewater is controlled, hence denitrifying bacterium can always propagate and function suitably. The time for activity recovery of denitrifying bacterium is not required. The repetition of wastewater treatment can be carried out without delay. The stability of wastewater treatment is improved, as the precision of controlling oxygen supply is improved. The method can be carried out with simple installation. The continuous monitoring of ammonia and nitrite nitrogen is not required.    DETAILED DESCRIPTION - Wastewater treatment method, involves oxidation treatment of supplying oxygen to wastewater in the presence of nitrifying bacteria and denitrifying bacterium, and converting ammoniacal nitrogen contained in wastewater into nitrite nitrogen using the nitrifying bacteria, and denitrifying process of converting nitrite nitrogen and ammoniacal nitrogen into nitrogen gas using the denitrifying bacterium. The oxygen demand (Qa) of the oxidation treatment and oxygen demand (Qo) which the oxidation of the organic matter contained in wastewater requires are calculated, based on the ammonia nitrogen and organic matter contained in wastewater. The supply of the oxygen is performed at a fixed supply rate which can maintain the average dissolved oxygen concentration of wastewater to 0.1 mg-O2/l or less. When the dissolved oxygen concentration of wastewater is raised by supply of the oxygen, based on the comparison with the oxygen supply total amount Y supplied to the wastewater, and the sum (Qt), the supply stoppage of oxygen or supply rate reduction is determined, where the sum (Qt) is the sum of Qa and Qo. An INDEPENDENT CLAIM is included for apparatus for wastewater treatment.