Included are a Zener diode D2 having a cathode connected onto a voltage output line, dividing resistors having one terminal connected to the anode of the Zener diode D2 and having the other terminal grounded, a transistor Q1 having a base connected to the junction point between the dividing resistors via a resistor and having an emitter grounded, a transistor Q2 having a base connected to the junction point between the dividing resistors and having an emitter grounded, and a microcomputer having an undervoltage detection terminal 10a which is connected to the collector of the transistor Q1 and to which 3.3 V is externally applied and a power supply terminal 10b which is connected to the collector of the transistor Q2 and via which a P-ON-H signal of 3.3 V that starts a power supply circuit 126 when the power supply is turned on is transmitted.