To prevent the waste of a battery by selecting a lowest power supply voltage from a DC-DC converter corresponding to the result of automatically detecting the state of an output voltage and supplying it to an output control circuit.

The output control circuit 20 composed of an error amplifier 21 and an output amplifier 22 whose power supply is the battery cascade connected to it is operated so as to turn a difference between a voltage obtained from a reference power supply 10 and a feedback voltage to zero and the voltage Vo or a current Io is taken out from terminals 41 and 42 and supplied to a load 80. At the time, when the voltage Vo supplied from the output control circuit 20 exceeds a reference voltage Va1 or Va2, a comparator 71 or 72 is respectively operated and the output of the DC-DC converter 62 is supplied to the output amplifier 22 as the power supply instead of a battery power supply by the output signals. Thus, the battery is not wasted unnecessarily and efficient output control is performed.