PURPOSE: To conduct accurate measurement of humidity invariably without any variation with time, by reading the humidity of air means of a microcomputer operation using an ultrasonic wave.

CONSTITUTION: A vibrator 3 of a sensor unit 1 is employed for the transmission side and a vibrator 4 thereof for the reception side. The vibrator 4 is excited by a high-frequency current from an ultrasonic oscillation circuit 18, and a vibration frequency signal thereof is detected by a detector 20 and inputted to MPU 23 through a filter 21 and an A/D converter 22. In the MPU 23 the ratio between a received energy from the vibrator 3 and a vibration energy is computed. Meanwhile, a signal from a temperature-sensing meter 8 is inputted to the MPU 23 through a linearizer 24. Based on each input signal, the MPU 23 leads out a humidity signal and supplies the same to a relative humidity indicating element 25 or an absolute humidity indicating element 26 via a switching element 27.