PURPOSE: To suppress the drop of impedance caused by the self-resonance of a capacitor, and to obtain the prescribed filter characteristic over a wide range by providing a resistor grounded in series to the capacitor on the respective both ends of plural pieces of coils connected in series, in the equalizing filter utilized for a high speed optical receiver, etc.

CONSTITUTION: Resistors 19, 20 and 21 are provided in series to capacitors 13, 14 and 15, respectively. Even in the case a self-resonance of parasitic inductance 16, 17 and 18 of the capacitors 13, 14 and 15 are generated, and the impedance of the capacitors 13, 14 and 15 drops, the impedance of the resistors 19, 20 and 21 is not lost, and a drop of the impedance of an (a) point, a (b) point and a (c) point can be suppressed. In such a way, in the case a prescribed equalizing output signal is led out to an input signal, since the drop of the impedance of the (a) point, the (b) point and the (c) point can be suppressed, that which is near an ideal frequency characteristic (k) is obtained as indicated with a dotted line (j), no distortion is generated in an output waveform and the equalizing filter in a wide area can be realized.