It is not always easy to keep high efficiency of nitrogen removal in a small-scale oxidation ditch, because its inflow rate generally changes significantly over time, especially for public sewerage facilities without a flow equalization tank. Based on experiments in a real oxidation ditch, a method of combining a DO meter and a timer was found to be useful for controlling intermittent aeration. In this method, a DO set value was used to stop the aerobic condition, and a timer set value was used to keep the anaerobic condition. The DO and timer set values effective for both nitrification and denitrification were about 0.5 mg/l and 1 hour, respectively. A T-N removal efficiency of approximately 90% was achieved under the most effective condition. It was supposed that the effective DO set value was influenced by the installation position of the DO probe and the total capacity of aerators per volume of the aeration tank. However, this study suggested that the DO set value could be determined by finding the rapid rise of the DO concentration, because it rose rapidly as soon as nitrification was completed.