To lessen a power loss of a battery in a power circuit wherein an AC adaptor and the battery is selectively connected to a load.

When an AC adaptor 4 is not connected, a transistor 7 becomes an ON state by control signals 102, 103 from a general controlling section 6. Therefore, voltage to be supplied to a DC/DC converter 5 can be increased and power consumption can be reduced by reducing a forward voltage drop (about 0.3V) due to a diode 2 to collector-to-emitter voltage (about 0.1V) of the transistor 7. Furthermore, an appropriate amount of power can be supplied according to a change in a load by controlling the base current of the transistor 7 by the control signals 102, 103.