PURPOSE: To prevent a frequency change due to humidity with simple constitution by inserting a fixed resistor in parallel with a variable resistor.

CONSTITUTION: A power voltage applied to a power terminal 1 is fed to a high frequency oscillator 2 and fed to a variable resistor 4 and a fixed resistor 8 via a constant voltage device 12. The power voltage divided by the fixed resistor 8, the semifixed resistor and the variable resistor 4 is fed to an intermediate terminal of a varactor diode 9 via a resistor 10 and a capacitor 11 for high frequency filtering. Since the resistance of the fixed resistor 8 is unchanged due to humidity, the voltage change at a point C between the variable resistor 4 and the semi-fixed resistor 7 is decreased and the change in the bias voltage by the resistance change of the variable resistor 4 is decreased.