To save power consumption at operation of equipment and to reduce electricity costs.

A power MOS-FET15 on main power source side is connected between a main power source 12 and a load 17, and a power MOS-FET 21 on battery side is connected between a battery 19 and the above load 17, and a charge circuit 18, which charges it when the voltage of the above battery 19 falls under a specified value, is connected between the above main power source 12 and the above battery 19, and a current sensor 14 is connected between the above main power source 12 and the above power MOS-FET15 on power source side, and a control circuit 23 is connected to this current sensor 14. This current sensor turns on the above power MOS-FET15 on main power source side and also turns off the above MOS-FET21 on battery side when the supply power to the above power MOS-FET15 on main power source side is over a specified value, and turns off the above power MOS-FET15 on main power source side and turns on the above MOS-FET21 on battery side when the supply power to the above power MOS-FET15 on main power source side is over a specified value.