Long term surveillance of natural O3was performed using an In2O3-based semiconductor O3sensor and the ultraviolet absorptionmethod ( U V-method). Qualitatively the indication of the semiconductor O3sensor has agreed well with the reading of the UV-method for more than 2 years without any readjustment after the initial adjustment. But quantitatively some discrepancy between them appeared in winter and in summer. This may be attributed to the absolute humidity dependence of the sensor. In spite of the discrepancy, the indication of the sensor was in good agreement with the reading of UV-method within 30% throughout the year. Consequently the semiconductor O3sensor showed an excellent long-term stability and a good reliability in the extremely low concentrationrange below 50 ppb under various circumstances. To obtain moreaccuracy, absolute humidity compensation was tried using a thermalconductivity type absolute humidity sensor.