The development of a CO2 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 CO2 sensor proposed here can monitor the CO2 concentration, which equilibrates with water vapor, permitting the detection of CO2 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 CO2 concentration in a frequency region above 100 Hz. A log-log plot of impedance versus M concentration obtained at the frequency of 100 kHz showed a good linear relationship in the concentration range between 3 x 10(2) and 1.5 x 10(5) ppm under as high a relative humidity as 80%. The response curve of the ocomposite film to CO2 concentration was not affected by the presence of NH3 and HCl in the concentration regions below 1000 and 10 ppm, respectively. Furthermore, no effect of coexisting gases such as O-2, and N2O was observed at all for the linear relationship of the log-log plot of dc resistance versus CO2 concentration.