Power supply device

An electric power is supplied from a commercial power source (12) to an input terminal (10). An electric load(13) is connected to an output terminal (11). A power storage (25) device is charged by the electric power supplied from the input terminal (10). The power storage device (25) supplies the electric power to the output terminal (11). A controller (40) monitors a voltage of the commercial power source (12), which is applied to the input terminal (10), and a state of charge of the power storage device (25). Based on the monitored result, the controller (40) changes a state of at least one primary battery (30) out of multiple primary batteries to an operation state enabling power supply to the output terminal (11). Furthermore, the controller (40) determines whether an operation of the operated primary battery (30) is good or poor, and changes a state of another primary battery to the operation state, if the operation state is determined to be poor.