A simple ionization absolute humidity gauge was developed in the present study. It consisted of a parallel plate ionization chamber with a radioactive alpha-ray source for supplying ion pairs to measure absolute humidity in air and an operational amplifier circuit for measuring the ionization current of the order of 10-12 A produced by alpha-particles. A sealed alpha-ray soure, 241Am, was used. The size of the gauge was 50 mm in diameter and 130mm in length. The distance between the two electrodes in the chamber was set at 15 mm which was enough longer distance than the maximum range of attenuated alpha-particles from the source and the wall effect on the ionization current was removed. The ionization current of the gauge was measured at various humidities in a range from 0 g/m3 to 25 g/m3 and temperatures in the range from 20°C to 40°C under the atmospheric pressure. It was shown that the ionization current is inversely proportional to the absolute humidity because a decrease in the ionization current is caused by the absolute amount of water vapor contained in the air. From the relationship between the absolute humidity and the ionization current, it was found that the absolute humidity could be measured with a sensitivity of 0.4 g/m3 that is equal to 3% in relative humidity at 20°C. The continual measurement of absolute humidity was carried out to confirm the performance of the gauge.