PURPOSE: To eliminate an effect of humidity for a semiconductor high frequency oscillator, by measuring the humidity of an oscillating part and then giving a compensation to the oscillating part from a relation among the measured value, the humidity stored previously and the oscillation frequency.

CONSTITUTION: A humidity detector 3 measures the humidity of a high frequency oscillator 1 and gives the measured value to an A/D converter 4. A frequency compensator 6 compares the humidity compensating value stored previously in a storage device 5 with the output value of the converter 4 and then delivers the quantity of compensation after an operation. The quantity of compensation is converted into an analog quantity through a D/A converter 7 and applied to the oscillator 1. As a result, a change of frequency due to the humidity can be compensated.