PURPOSE: To reduce the deterioration in the frequency stability due to frequency secular change by updating original compensation data while monitoring secular characteristic change.

CONSTITUTION: When a phase locked loop network and a VCO 3 synchronously with a highly stable reference frequency signal are synchronized with each other, a CPU 4 tries to calculate an external control voltage required to generate a center frequency under the environmental condition depending on the detection result of ambient temperature and a power supply voltage by sensor sections 6,7, compares the result of trial calculation with an actual external control voltage to detect the difference. The detected difference is discriminated to be produced due to secular change in each component of the VCO 3 and the phase locked loop network. When the phase locked loop network causes out of synchronism from a highly stable reference frequency signal and transits to the self-running state, the CPU 4 reads data in response to a current external control voltage, a power supply voltage and ambient temperature from the storage device 5 and sends the data to a compensation voltage generating section 9 to apply a voltage signal for compensation to an internal control terminal 11 of the VCO 3.