The concentration of NO2 gas, an environmental pollutant. can be reduced by photocatalysis. A simple method for measuring the efficiency of photocatalysis is required. In general, the efficiency of photocatalysis is determined by measuring the decrease in the concentration of NO2 gas during UV light irradiation. In this study, an NO2 gas sensor made of ITO film was used to detect the photocatalytic oxidation of NO2. When ZnO film prepared by electroplating was used as the photocatalyst, the photocatalytic reaction was measured continuously with good reproducibility, showing that this method is useful. Next, the photocatalyst was changed from ZnO to ITO film, and the phstocatalytic efficiency was studied. ITO film composed of tine particles (30-60 nm), which was prepared by magnetron sputtering, had a very low efficiency. However, ITO film with ITO whiskers (30 nm in diameter and 500-700 nm in length) prepared by die electron shower method showed a similar high efficiency to ZnO film.