Abstract: A reference voltage is applied from a reference voltage generating circuit to the non-inverting input terminal of an amplifier for supplying a drive voltage to the gate terminal of an NMOS transistor, and the output voltage appearing at the source terminal of the NMOS transistor is divided by a resistor pair and applied to the inverting input terminal of the amplifier. The voltage obtained by adding a voltage equal to or higher than the voltage for sufficiently driving the NMOS transistor to the output voltage appearing at the source terminal of the NMOS transistor is generated by a charge pump circuit and supplied to the amplifier as a power supply voltage. With this configuration, the drive voltage for the NMOS transistor is suppressed to the required minimum voltage while the drive voltage is obtained securely. The power consumption in the amplifier can thus be suppressed.