PURPOSE: To protect a battery from being burdened with an extra load by sustaining ON state of a second diode in battery circuit so long as power is fed from the battery to a load.

CONSTITUTION: If power supply from a DC power supply PS is interrupted when power is fed from a battery B to a load L, current passing through a resistor R2 is interrupted and the gate potential of a second P-channel enhancement type FET transistor Q2 drops to earth potential thus turning the transistor Q2 ON. Since the resistance of the transistor Q2 is lower than the forward resistance of a second diode D2, power is fed from the battery B through the transistor Q2 to the load L. At that time power supply from the battery B to the transistor Q2 for transistor Q2 in ON state is not required. According to the constitution, the battery B is protected against being burdened with an extra load.