PURPOSE: To improve distortion properties or noise properties and to eliminate the need for an eliminating circuit of an energy diffusing signal, by adding the output of a coefficient device which multiplies a prescribed coefficient by an AC component to a DC component of a DC component detector through an adder to supply it to a voltage controlling oscillator.

CONSTITUTION: An LPF19 which extracts an AC component is controlled to the output side of a DC inverting amplifier 10 via a capacitor 18. A wave detector 20, a coefficient device 21 and an adder 15 are connected to the output side of the LPF19. A DC inverting amplifier 22 and an LPF23 are connected to the output side of the adder 15. Thus the DC component of the output voltage of the amplifier 10 is K-multiplied by the envelope curve of the AC component of the output voltage to be supplied to a voltage controlling oscillator 3. As a result, the local oscillating frequency of the oscillator 3 has fluctuation. In this connection, the lower frequency of an intermediate frequency signal fIF is set to be a prescribed frequency by setting the coefficient K at a proper value. As a result, the frequency band of the signal fIF is always kept constant.