PURPOSE: To improve voltage drop in comparison with the use of a diode by using a transistor switch for preventing backup current from flowing backward to another circuit at the breaking of electric power for operation.

CONSTITUTION: Electric power for operation is supplied to a memory 10 from a normal electric power source 12 through a switch 14 and a transistor 16. Simultaneously, the electric power source for operation applies electric power to another circuit 20 through a line 18. When the switch 14 is turned off, the transistor is also turned off. At that time, current from a backup electric power source 24 which has been charged by the power source 12 for normal operation or is prevented from current leakage is applied to the circuit 10 through a backup power source charging resistance or inverse current checking diode 26. The backup current does not flow into the other circuit 20 because the transistor 16 is in an off state.