A humidity sensor as well as a humidity detection circuit are disclosed, the former employing a humidity sensitive material the electric resistance of which varies in response to the variance of humidity in an atmosphere and the latter employing such humidity sensitive material. The humidity sensor comprises a base plate made of aluminum or the like with a humidity sensitive material formed on the base plate so as to form a film of a high molecular polymer unit which contains a reactive cationic monomer unit, is capable of detecting humidity over a wide range shows little variance in nature in a high humidity atmosphere of organic gas, and has a long life. The humidity detection circuit comprises an oscillation circuit, a pair of electrodes which are connected by means of the humidity sensor and to one of which is supplied the output of the oscillation circuit as an input, and an amplifier circuit which is connected to the other one of the pair of electrodes. The humidity detection circuit is advantageous in that it can be used for a long period of time without varying the characteristics thereof.