To easily downsize the whole circuit by inserting a field effect transistor between a regulator and a battery, and charging the battery and discharging to the regulator through the field effect transistor.

A power circuit 1 is equipped with a constant current circuit 2 as a regulator, and it controls the current supplied from a DC power source 3 and supplies load with a current of fixed value. Moreover, it is equipped with a battery B for backup for the time of power failure, etc., and it is connected to a constant current circuit 2 through a field effect transistor R. Then, when voltage is supplied from a DC power source 3, the line between the source and drain of the field effect transistor F becomes electrically continuous, and the battery B is charged. Moreover, when the voltage supply from the DC power source 3 is broken by power failure, etc., the electric continuity occurs between the drain and the source of the field effect transistor F, and electricity is discharged from the battery B to the constant current circuit 2. As a result, there is no necessity to provide a diode for discharge of battery, so the downsizing of the circuit is materialized easily.