PURPOSE: To improve an S/N ratio and resolution, by synthesizing the output of an oscillating circuit using an electrostatic-capacity-variation type humidity sensor and the output of reference-frequency oscillation so as to form an inter- mediate frequency signal, detecting the humidity, thereby electrically increasing the slight amount of variation.

CONSTITUTION: A signal of a frequency corresponding to the variation in electrostatic capacity is outputted by an electrostatic-capacity humidity sensor 1 and an oscillator 2. The signal and the output signal of a reference oscillator 3 are synthesized by a mixer 4. The intermediate frequency signal, whose variation in electrostatic capacity is slightly increased is electrically formed. The humidity is detected hightly sensitively at an excellent S/N ratio and at high resolution through a filter 5.