An integration circuit includes a differential amplifier constituted by at least two bipolar transistors serving as amplifying elements, a capacitor connected, as a load, across the collection electrodes of the differential amplifier, and a field-effect transistor having source and drain electrodes connected between the emitter electrodes of the two bipolar transistors. A control voltage is applied to the gate electrode of the field-effect transistor. By changing the resistance value between the source and drain electrodes of the field-effect transistor using a gate voltage, the transconductance of the differential amplifier is changed over a wide range. As a result, the time constant of the integration circuit is changed, such that if the integration circuit is used for an active filter, for example, the cut-off frequency can be changed by changing the time constant of the integration circuit.