PURPOSE: To attain a filter circuit in which a frequency band of an output signal is substantially constant regardless of a frequency characteristic to an input signal by providing an arithmetic circuit controlling the capacitance of a variable capacitive element of a variable low pass filter or the like to the filter circuit.

CONSTITUTION: The capacitance of a variable capacitor 13 is varied with a control voltage applied from an arithmetic circuit 8 to vary the frequency characteristic of the low pass filter 1. That is, suppose that band characteristic with respect to a control voltage is as shown by the curve (a) in waveform diagram, the frequency characteristic is changed as shown by the curve (b) by decreasing the capacitance of the capacitor 13 and as shown by the curve (c) by increasing the capacitance of the capacitor 13. An input of buffer circuits 2, 3 is connected to an output of the filter 1 and an output of the circuit 2 is connected to one input of the circuit 8 via a narrow band pass filter 4 and a peak hold circuit 6. Moreover, an output of the circuit 3 is connected to the other input of the circuit 8 via a narrow band pass filter 5 and a peak hold circuit 7. The circuit 8 generates a control voltage to make the ratio of a couple of input signals constant to apply feedback control to the capacitor 13.