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Niobium oxide films were prepared over a quartz plate by withdrawing it from a solution of niobium penta-isopropoxide (NIP) dissolved in sec-propyl alcohol. The films calcined at 673 K were well controlled in the film thickness either by NIP concentration in the solution or by the withdrawing rate of the quartz plate from the solution. BET surface area of the calcined films was as huge as around 400 m2g−1 and was not reduced by cyclic adsorption/desorption of water vapor. A rapid decrease and increase in the electrical resistivity of the calcined films was well associated with the cyclic adsorption/desorption of water vapor. The decrease in the electrical resistivity of the films by water vapor adsorption was more than 10 times sensitive than the decrease caused by the adsorption of ethanol, hydrocarbons, carbon monoxide and carbon dioxide. These results suggest an application of the niobium oxide films as an element of a humidity sensor. The calcined films were proved by SEM observation to consist of tiny particles possessing a lotofmicropores sized less than 20 A. The decrease in the electrical resistivity of the films, or the increase in the electrical conductivity, was attributed to the water vapor adsorbed in these micropores. In order to identify the charge carriers during water vapor adsorption on the films, changes in the impedance and the phase shift caused by water vapor adsorption were measured using an LCR meter in AC frequency range of 10 Hz to 100 kHz. From a complex impedance plotting, single semicircule was obtained for water vapor adsorption onto the films, suggesting single adsorbed species as a charge carrier. Assuming an equivalent electric circuit for the films adsorbing water vapor, a constant capacitance was calculated under various partial pressures of water vapor, probably suggesting that the charge carriers will be H3O+ on the films.