A detector for continuous detennination of a trace amount of aqueous ozone, utilizing an ozone extraction process from liquid (water) into the gas phase and an In2O3thin film semiconductor ozone gas sensor, has been developed. It offers many advantages including high sensitivity to aqueous ozone and high aqueous ozone selectivity in the presence of various kinds of interfering compounds in water. It is able to detect 0.5 ppb of aqueous ozone with a sufficient response. The response to 1 ppm of free chlorine in water was smaller than that to 10 ppb of aqueous ozone. Long term monitoring aqueous ozone over 100 days was performed in an experimental plant for ozone treatment in water supply with the present aqueous ozone detector. The readings of the detector agreed well with the results of iodometry during the monitoring.