PURPOSE: To utilize plural kinds of auxiliary power supplies effectively according to the purposes of using, by changing over so as to use the auxiliary power supply having the higher initial voltage than other ones firstly, monitoring the voltages of plural auxiliary power supplies having different voltages by a voltage comparator.

CONSTITUTION: When a main power supply 14 is broken, the voltages of auxiliary power supplies 15-17 are compared by a voltage comparator 13 and an auxiliary power supply 15 having the highest voltage is selected. The auxiliary power supply 15 is connected with a load 11, by changing over a changing over switch 12 according to the comparison result. When the voltage of the auxiliary power supply 15 is lowered due to its dissipation of energy, an auxiliary power supply 16 having the next highest voltage is selected by the voltage comparator 13 and the auxiliary power supply 16 is connected with the load 11, by changing over the changing over switch 12. Also, the auxiliary power supply 16 dissipates its energy and its voltage is lowered, an auxiliary power supply 17 is selected and connected with the load 11 similarly. The changing over switch 12 and the voltage comparator 13 can be replaced with a logical sum operated by diodes.