The development of a CO 2 sensor capable of operating in a highly humid atmosphere is desirable in order to preserve a clean environment in an airtight room, to apply to medical equipment such as a metabolic breathing system, etc. The CO 2 sensor proposed here can monitor the CO 2 concentration, which equilibrates with water vapor, permitting the detection of CO 2 under high humidity. The sensing material was a composite film consisting of base-type poly(anthranilic acid) (PANA) and poly(vinyl alcohol) (PVA). The ac impedance was enhanced with increasing CO 2 concentration in a frequency region above 100 Hz. A log-log plot of impedance versus CO 2 concentration obtained at the frequency of 100 kHz showed a good linear relationship in the concentration range between 3x10 2 and 1.5x10 5 ppm under as high a relative humidity as 80%. The response curve of the composite film to CO 2 concentration was not affected by the presence of NH 3 and HCl in the concentration regions below 1000 and 10 ppm, respectively. Furthermore, no effect of coexisting gases such as O 2 and N 2 O was observed at all for the linear relationship of the log-log plot of dc resistance versus CO 2 concentration.