To provide a humidity sensor element which is superior in water resistance, solvent resistance and gas resistance so that moisture in an atmosphere can be detected and quantitatively determined, which eliminates a hysteresis in a wide humidity region and which has a stable output characteristic and to provide a method of manufacturing the humidity sensor element.

A humidity sensor comprises a pair of electrodes so as to be faced via a gap on an insulating substrate, and it comprises a moisture sensitive membrane on the gap. A contamination and/or an oxide in the uppermost layer of the insulating substrate are removed by a physical means, the insulating substrate is coated with a monomer having an ethyle unsaturated reaction group, and the monomer is polymerized on the insulating substrate so as to form the moisture sensitive membrane.