PURPOSE: To obtain an active filter not affected by a parasitic capacitance of a transistor (TR) to a built-in capacitor with less number of elements by controlling a DC current flowing to a resistive impedance component to change its impedance thereby changing the time constant of the filter.

CONSTITUTION: The resistive impedance component in the resistive impedance component and a capacitive impedance component forming an active filter is formed by equivalent resistances 33E, 34E of diodes 33, 34 and an emitter internal equivalent resistance 32E of a TR32 and a DC current flowing to the equivalent resistors 32E∼34E is changed to change the time constant of the filter. Through the constitution above, since the time constant of the filter is changed without using an operational amplifier as a conventional filter, the increase in the number of circuit components is avoided even in forming a high-order filter.