Abstract: The gate terminal of a FET is supplied with a bias voltage from a gate biasing circuit and a fluctuation in voltage V.sub.CS applied to a constant-current circuit is detected by a voltage-fluctuation detector. A gate-bias controller controls the gate biasing circuit to lower the gate bias voltage in a case where the fluctuation in voltage V.sub.CS has changed in an increasing direction, and to raise the gate bias voltage in a case where the fluctuation in voltage V.sub.CS has changed in the decreasing direction. Further, there are provided a plurality of constant-current sources having respective FETs and biasing circuits for setting a prescribed power-supply voltage characteristic and a prescribed temperature characteristic. A constant-current circuit is constructed by parallel-connecting the FETs in the plurality of constant-current sources through the drains and sources thereof.