An electronic device having a sub battery supplying operational power on separation of a main battery is provided to plan convenience of a user when the main battery is replaced and keep working contents of an operated system by preventing a unpredictable system turn-off situation. The sub battery(3) supplies the operational power to the system(1) in case that the main battery(2) is separated for replacement during operation of the system. A power detector(4) measures sub power supplied as the operational power by the sub battery. An alarm part(5) informs the user of measured sub power information received from the power detector. A power controller(6) makes the sub battery supply the operational power to the system in the case that the main battery is separated for the replacement during the operation of the system.
