Electric conductivity of polyaniline thin film formed by r.f. (1 MHz) plasma excitation changes in exposure to CO2 gas. The polyaniline thin film (ca 2000 Angstrom thick) is set between parallel plates of Au thin film electrode of which one side of the electrode used the obliquely deposited film (ca 180 Angstrom thick) to allow passage of the gas into the polyaniline thin film. Current measurement between the electrodes biased with 1.5 V lithium cell was carried out for the sensor being kept in a tubular space (ca 30 cm(3)) while the gas flow (1 similar to 2.0 l/min) is in the condition of 'ON' to 'OFF'. The film conductivity of the sensor with an active area of 34.3 mm(2) increases approximately six times in magnitude, which is proportional to the area and to the level of the flowing gas. The data show that the changes in current are reversible and respond rapidly below 1 s.