Abstract: In an intermittent aeration activated sludge process in which aeration and agitation is repeated alternately, a first aeration tank and a second aeration tank are connected to each other and an ORP meter is applied to each tank. In the first aeration tank, the sum of an aeration period and a denitrification period of an agitation step is controlled to a predetermined period T.sub.gs based on a time when a bending point appeared on an ORP curve in a previous cycle. In the second aeration tank, the sum of an aeration period and an agitation period is controlled to a predetermined period T.sub.ds longer than T.sub.gs based on a time when an ORP measured value reached a predetermined value in a previous cycle. Operations of the first and second aeration tanks are simultaneously transferred from the agitation to the aeration based on detection of the predetermined ORP value. As a result, a necessary anaerobic condition period is secured in the first aeration tank.