A constant-voltage circuit includes a first transistor, a first control circuit, and a second control circuit having a second transistor and a differential amplifier. The first transistor controls an output current according to a first control signal output by the first control circuit such that an output voltage is substantially equal to a predetermined voltage. The second control circuit has a response property faster than the first control circuit to a variation of the output voltage, and causes the first transistor to increase the output current for a predetermined time period, regardless of the first control signal, when the output voltage varied to an extent greater than a predetermined output voltage variation value. The second transistor controls an operation of the first transistor according to a second control signal output by the differential amplifier such that a voltage at an inverting input terminal is substantially equal to the bias voltage.