The application of ozone to waste water treatment in color laboratories were investigated with small-scale pilot apparatus combined with a small biodegradation plant. The combination enhanced the degree of degradation, since ozone decomposed un-biodegradable oxygene demanding components remaining in the effluent from the biodegradation tank. However, it was also found that insufficient biological treatment caused the decrease in efficiency of decomposition by ozone. The limit of COD level attainable in overall process was 20 mg/1, although 10 mg/1 was also possible in a particular deliberate case. The ozone treatment in the combined process is similar to the tertial treatment in the municipal sewage treatment plant, however, ozone consumption in this process was much larger than those for the latter.