PURPOSE: To stabilize an oscillating frequency regardless of external causes by obtaining the fluctuation part of the oscillating frequency of a second voltage controlled oscillator(VCO) with a phase compared result and suppressing fluctuation in the oscillation central frequency of a first VCO.

CONSTITUTION: When a power source is inputted, a prescribed oscillating frequency signal is outputted from a VCO3. When a frequency center is fluctuated, the rectangular wave signal of the fluctuation part is outputted from a phase comparing mean 4. This signal is smoothed by an LPF5 and caused to be a direct current voltage V. Then, the signal goes to be a prescribed level and becomes the correcting level of the oscillating frequency of VCO3 and VCO2. When an input signal signal is given to the VCO2, the oscillation of the central frequency to correspond to the input signal is executed from the VCO2 and the correcting value V is added through an LPF7. Accordingly, the oscillation of the central frequency to correspond to the correcting value and input signal is executed in the VCO2. Since the correcting value is changed by the various external causes and changed to a value which causes the oscillation central frequency of the VCO2 to be constant, the oscillation of the VCO2 is made stable.