A voltage difference is developed between the negative input side 2c and the negative output side 2d of the voltage boost circuit 2, the negative input side 2c is connected to the negative side of the power source 5, and the negative output side 2d is connected to the negative side of the rechargeable batteries 1. Further, the load FET 3 is connected between the negative input side 2c and the negative output side 2d of the voltage boost circuit 2, and an n-channel FET is used as the load FET 3. The control circuit 4 maintains the load FET 3 in the OFF state to cut-off conduction when there is no power outage, and switches the load FET 3 ON to supply power to the load 12 from the rechargeable batteries 1 during a power outage.