PURPOSE: To suppress frequency fluctuation caused by the rapid temperature change of a voltage controlled oscillator.

CONSTITUTION: The output of a voltage controlled oscillator 1 is branched by a power divider 2, one output is used as an oscillator output, and the other output is utilized as the temperature corresponding data of a resonator in the voltage controlled oscillator 1. The output frequency used as the temperature corresponding data is outputted as a temperature display voltage to express the temperature of the resonator by a frequency/voltage conversion circuit 3. This temperature display voltage is supplied to a temperature compensating circuit 4 and outputted as a control voltage to suppress the frequency fluctuation caused by the temperature change of the voltage controlled oscillator 1. Since this control voltage is impressed to the oscillation frequency deciding element such as a variable capacity diode in the voltage controlled oscillator 1, speedy temperature compensation is performed.