PURPOSE: To detect the humidity by applying a direct current even in case when an existing temperature sensor is used, by applying a DC voltage to the humidity sensor of an electric resistance variable type, and thereafter, measuring the resistance value of the sensor after a prescribed time, and detecting the relative humidity.

CONSTITUTION: The DC voltage is applied to the sensor, and thereafter, the resistance value of the sensor is measured after the prescribed time within 20sec. In such a way, if only within 20sec, the polarization is comparatively small, and also the resistance value corresponding to the relative humidity is shown with a high reproducibility, therefore, even in case when DC is applied, a stable humidity detecting characteristic is obtained, and the humidity can be detected by DC driving which has been impossible up to the present.