PURPOSE: To reduce the size of a structure by energizing a load with a second battery when the output voltage of a DC/DC converter to be energized by a first battery becomes less than a predetermined value.

CONSTITUTION: When the voltage Vcc of a DC/DC converter 7 reaches 5V and a reset generator 24 outputs a signal of high level to a line 28, a common contact 14 of switching means 9 is conducted to individual contacts 11, and an output VDD is produced at the output of the converter Vcc as it is. Thus, a memory 2 is energized by the output of the converter 7. If the remaining capacity of a battery 4 becomes small, the BD signal of a battery voltage detector 21 becomes a high level. Thus, the means 9 outputs a voltage VLi to be output to a line 19 through a voltage drop circuit 18 from a battery 5 as an output VDD. Thus, the memory 2 is backed up by the battery 5, and continuously energized.