SnO2- and In2O3-based thin films were prepared by magnetron sputtering onto alumina ceramic substrates, and their sensitivities to NO, and NO were investigated. The deposition of oxide thin films varied, depending on the substrate temperature, sputtering rate, and oxygen partial pressure, When the SnO2 sensor doped with Nb2O5 and TiO2 and prepared under optimal conditions was kept at 300 degreesC, its sensitivity (RgRair) reached a value of 2 or more in an atmosphere including 0.4 ppm NO2. The In2O3-based thin film sensor including appropriate amounts of additives showed a much higher sensitivity, which was satisfactory to detect 0.04 to 0.06 ppm NOx, as required by the Ambient Environmental Quality Standard. The detecting mechanism is discussed based on a previously reported theory.