PURPOSE: To change frequencies of pass band or block band by using a voltage controlled oscillator as a variable frequency component.

CONSTITUTION: An input signal is fed to a mixer 11, where the signal is mixed with an oscillation output of a voltage controlled oscillator 12 whose oscillation frequency is changed at a high speed by changing a voltage fed to a control terminal 16 at a high speed to apply frequency conversion. Only output signals whose frequency passes through a band-pass filter 13 among those from the mixer 11 reach a mixer 14. The signal reaching the mixer 14 is mixed with the oscillation output from the oscillator 12 having the same frequency as the signal fed to the mixer 11 The output signal of the mixer 14 is given to a filter 15 to eliminate an undesired signal and the result is outputted to an output terminal 2. Thus, the oscillating frequency of the oscillator 12 is changed in response to the voltage fed to a control terminal 16 to change the frequency of the signal passing through the titled variable filter.