PURPOSE: To attain the sophisticated gain control with high accuracy by adjusting the duty ratio of a pulse signal so as to control the on/off of a switch means switching alternately an integration resistor between an output of an operational amplifier and a signal common potential.

CONSTITUTION: A couple of switches S1,S2 switching alternately the integration resistor A between an output E0 of the operational amplifier A1 and the signal common potential are provided. A duty ratio variable circuit l is provided, which adjusts optionally the on/off duty ratio of the switching of the switches S1,S2. When the circuit 1 outputs a pulse signal of a high level, the switch S1 is turned on, while a low level pulse signal is outputted, the switch S2 is turned off. Thus, the sophisticated gain control with high accuracy is attained.