A humidity sensor includes a pair of interdigital electrodes disposed on an insulating substrate and defining a gap therebetween, an undercoat layer of silane compound lying on the gap-defining electrodes and substrate, and a humidity sensitive thin film lying thereon. The humidity sensitive thin film is formed of a crosslinked product of a conductive polymer having ethylenically unsaturated groups, and physically bound to the undercoat layer through an interpenetrating polymer network. The sensor is fully resistant to water so that it ensures stable operation over a long time even in a dew condensing atmosphere. The sensor is free of hysteresis and able to measure humidity over a very wide range.