A constant-voltage circuit includes: first and second field-effect transistors; a first node connected to the drains of the first and second field-effect transistors; a second node connected to the gates of the first and second field-effect transistors; a bipolar transistor whose collector is connected to the second node; a resistor connected to the source of the second field-effect transistor and the collector of the bipolar transistor; and a bias circuit that is connected to the source of the second field-effect transistor and supplies a bias voltage to the base of the bipolar transistor, wherein a power supply is connected to the first node and a constant voltage is outputted from the source of the first field-effect transistor.