PURPOSE: To avoid the limitless charging to an internal power supply by a method wherein two semiconductor switching circuits which are connected in series to each other so as to have both the anodes of respective parasitic diodes connected to each other are used and, when a power is supplied to a load, the semiconductor switching circuit on a supply line side is turned off.

CONSTITUTION: When a power is supplied to a load 9 through a feeder line 3 from a DC power supply 1, a P-type MOS-FET 4 is turned OFF by a DC power supply voltage monitor circuit 7 and a P-type MOS-FET 5 is turned ON by a battery voltage monitor circuit 8. In this state, as the ideal MOS-FET 4a of the P-type MOS-FET 4 is in an OFF-state and the voltage of a battery 6 is lower than the output voltage of the DC power supply 1, the direction of a diode 4b in the P-type MOS-FET 4 is reverse, so that the charging to the battery 6 from the feeder line 3 can be avoided. Further, the overdischarging and the momentary interruption of the feeder line 3 can be avoided.