The output terminal of a first voltage-current conversion circuit, which includes operational amplifiers and a resistor, is connected to the output terminal of a second voltage-current conversion circuit which includes operational amplifiers and a resistor, and also to the negative input terminal of the second voltage-current conversion circuit via a source follower as an impedance conversion circuit which is formed by an NMOS transistor and constant current source. Furthermore, the output terminal of the first voltage-current conversion circuit serves as the output terminal of a differential amplifier circuit. The positive-phase input terminal of the second voltage-current conversion circuit is connected to a reference voltage. Since the node of the current outputs of the first and second voltage-current conversion circuits has a high impedance, these circuits operate to equalize their output currents.