In order to suppress scum formation of activated sludge accompanying the abnormal growth of Nocardioform Actinomycetes (Nocardia sp.) in the final sedimentation tank, etc., an experimental study was made by adding ozone (at ozone consumption 2-6 ml/l) in the aeration tank. The abnormal growth of Nocardia almost disappeared, and foaming in the aeration tank and scum formation of activated sludge in the final sedimentation tank were not seen, showing that the scum suppression effect of ozone addition was obvious. The treated water quality was little affected except for accelerated nitrification by ozone addition, and the sludge settleability was improved.