PURPOSE: To make the frequency lead-in range of a resonance circuit wide and the noise band narrow by providing a means which changes control signals so that the mesial magnitude width of the resonance circuit may be narrower according as the output of the resonance circuit increases.

CONSTITUTION: An input signal and the output of voltage control oscillator 5 are applied to the first multiplier 1, and the output of the first multiplier is applied to resonance circuit 600. The output of resonance circuit 600 and the output of the voltage control oscillator are applied to the second multiplier 2, and the input and the output of resonance circuit 600 are applied to phase difference detector 3. The output frequency of voltage control oscillatoer 5 is controlled by the output of detector 3, and a signal which has the same phase as the input signal is obtained from the output of the second multiplier. Resonance circuit 600 where the mesial magnitude width is changed by control signals is used as the resonance circuit 600 above, and control signals are changed so that the mesial magnitude width may be narrower according as the absolute value of the output level of phase difference detector 3 is reduced. Then, control signals are changed so that the mesial mamagnitude width of the resonance circuit may be narrower according as the output of the resonance circuit increases. As a result, the frequency lead-in range of resonance circuit 600 can be made wider, and the frequency stability can be made well.