PURPOSE: To set the entire filter characteristic to an optimum value by comparing an input signal level to a filter circuit with a signal level after passing through the filter, calculating a required bias voltage, converting the voltage into an analog voltage and feeding the result to the filter circuit.

CONSTITUTION: A control section 1-7 takes a difference between a signal level received from an A/D converter 1-6 and an input signal level to a predetermined filter circuit 1-3 to calculate the attenuation of the filter circuit 1-3. Then the bias voltage is calculated so that the attenuation is a preset value to control the bias voltage of the filter circuit 1-3. The said bias voltage is latched by a latch circuit 1-8, converted into an analog quantity by a D/A converter 1-9 and fed to the filter circuit 1-3. Moreover, when the Q of the MOSFET-C filter circuit 1-3 is set, a signal within a filter pass band and having a frequency close to the cut-off frequency is used as the control signal.