It is necessary to reduce the deterioration of treated water quality in ozone-based excess sludge reduction systems. In this report the treated water quality in an excess sludge reduction system using intermittent ozonation with highly concentrated ozone was investigated. This was proven to be a highly efficient way of reducing sludge in a previous consideration. From field experiments using real wastewater, the intermittent ozonation with highly concentrated ozone was found to be superior in the reduction of the DOC concentration increase in the treated water to the continuous ozonation with low concentrated ozone. Further investigation indicated that this feature was attributed to the organic substances decomposition activity of the activated sludge in the aeration tank, which was recovered. The continuous increase in the organic load of the aeration tank was avoided by securing the non-ozonation period realized by the intermittent ozonation. From these results the intermittent ozonation with highly concentrated ozone was concluded to be extremely effective for reducing the deterioration of treated water quality, while keeping the efficient sludge reduction ability.