An uninterruptible power source device 10 in the present invention includes: an input-output terminal 11 that is connected in parallel with a power source line 21 through which electric power is supplied from an external power source 20 to a loading device 30; a battery unit 12 whose rated voltage is lower than a voltage V1 of the external power source 20; a DC/DC converter 13 that converts a voltage V2 of the battery unit 12 into a voltage V3 equivalent to a difference between the voltage V1 of the external power source 20 and the voltage V2 of the battery unit 12; a charging circuit 14 that charges the battery unit 12 through the input-output terminal 11 with the electric power of the external power source 20; and a discharging circuit 15 that performs discharge to the loading device 30 through the input-output terminal 11 at a voltage resulting from adding an output voltage V3 of the DC/DC converter 13 to the voltage V2 of the battery unit 12, at the time of electric power interruption of the external power source 20.