Power supply apparatus with a backup time

In order for a backup battery (3) to prolong a backup time period for a mobile instrument, a control signal Sc1 becomes either low level or a high level in accordance with determination if a power supply voltage Vm of a main battery (2) is either lower or higher than a prescribed level V1 requiring backup of the backup battery (3). A control signal Sc2 also becomes either a low level or a high level in accordance with determination if a backup voltage Vb of the backup battery (3) is either higher or lower than a power supply voltage Vm supplied by a main battery (2). When both of the control signals Sc1 and Sc2 are in the high level, a control switch (5) is turned ON so as to stop a current from flowing from the backup battery (3) to the main battery (2). The control switch (5) is otherwise turned OFF.