A power source device capable of using both an external DC power source and a battery contained therein, and an electronic device using this power source device. A circuit is provided to measure the battery voltage, to calculate the remaining capacity of the battery and to indicate the remaining capacity. Means is provided which detects the state where no battery is mounted in the power source device, and short-circuits the input side of the battery voltage-measuring circuit. When no battery is mounted, therefore, a voltage leaking from the external DC power source is not erroneously measured as the battery voltage, and the remaining amount of power of the battery is correctly calculated.