PURPOSE: To detect the concentration of aqueous vapor in a high temperature gas eliminating any complicated attached electric circuit by checking a potential difference depending on a difference in the humidity between two sides of a partition made of a proton conductive solid oxide.

CONSTITUTION: A partition 13 made of a proton conductive solid oxide is mounted on the end of a ceramic tube 12 and provided with porous electrode 14 on both sides thereof 13 which are connected to a lead wire 15. A desiccating agent 16 such as synthetic zeolite is usually sealed into the ceramic tube 12 to keep the vapor pressure in the chamber constant. The proton conductive solid oxide herein used is basically made up of oxide of strontium and cerium to which oxide of at least one metal selected from yttrium, scandium, ytterbium, neodyium, magnesium, praseodymium and zinc is aded. When the humidity sensor thus obtained is exposed to the atmosphere of gas to be measured, an electromotive force is generated between the electrodes on both sides of the partition according to a partial pressure of the vapor in the gas.