PURPOSE: To prevent a hang-up phenomenon by using a frequency error detecting circuit which delivers a signal corresponding to the frequency error between an input signal and a reference signal and a voltage control oscillator which is controlled by the output of said error detecting circuit so that the frequency error becomes minimum, and produces the reference signal.

CONSTITUTION: The outputs of LPF16 and 18 are supplied to the input of double balanced modulators 12 and 13 and orthogonal to each other since a π/2 phase difference is given to the output signal of a voltage control oscillator 21. The output of the LPF18 has a 90° phase advance via a differentiator 22 and therefore both inputs have same phases or adverse phases at the input of a multiplier 23. Then a signal proportional to the difference frequency between the input signal and the output signal of the oscillator 21 is produced at the output of the multiplier 23. The output of the multiplier 23 is applied to the oscillator 21 as the control voltage via a loop filter 24 having a proper transmission function F(S). Thus the feedback control so that the minimum difference is obtained between an input frequency and the output signal frequency of the oscillator 21.