A multistage activated sludge process involving a predation effect caused by microorganisms realizes further improvement in treatment efficiency and further decrease in the amount of excess sludge generated while a stable quality of the treated water is maintained. This process includes a first biological treatment process for treating BOD in organic wastewater at a high load to convert the BOD to dispersed bacteria, and a second biological treatment process for forming the converted dispersed bacteria into a floc and allowing microorganisms to coexist. The second biological treatment process is performed under a condition of pH 5 to 6. Alternatively, an excess sludge treatment process for decomposing at least a part of sludge in the second biological treatment process and/or sludge obtained by performing solid-liquid separation of sludge in the second biological treatment process under aerobic conditions, and returning the treated sludge to the first biological treatment process and/or the second biological treatment process is performed under a condition of pH 5 to 6.