PURPOSE: To prevent an influence due to a leading-in phenomenon and to stabilize a frequency by providing first and second voltage controlled oscillators, and suppressing fluctuation of the oscillatiion central frequency of the first oscillator on the basis of the fluctuation part of an oscillating frequency to be obtained with a phase comparing means.

CONSTITUTION: A second voltage controlled oscillator(VCO)3 is operated by a timer circuit 4 in each prescribed time. In each prescribed time, the oscillation part of the oscillation central frequency of the VCO3 is obtained by a phase comparing means 5 and this fluctuation part is defined as a direct current voltage V by an LPF6 and held by a hold circuit 8 for a prescribed time. Since the fluctuation part V can be regarded as the oscillation part of the central frequency of the VCO2, it is added through an LPF9 to the input signal of the VCO2. Thus, the oscillation centrol frequency of the VCO2 is exactly controlled samelly as the input signal and oscillation fluctuation at the time of inputting power source or converting temperature can be suppressed. Since the fluctuation part of the frequency is obtained in each prescribed time, the influence of leading-in phenomenon can be prevented between the both VCOs.