An uninterruptible power supply unit essentially includes: a DC power supply source 3, a parallel converter 4, and a series converter 6 that are individually connected in parallel to both ends of an electrolytic capacitor Cdc having a pair of capacitors C1 and C2. An input terminal Pin1 is connected to an intermediate point of the electrolytic capacitor Cdc. A power supply line 1n interlinking an input terminal Pin1 and an output terminal Pout2 is connected to multiple switching elements of the parallel converter 4 via a reactor Lin. Likewise, multiple switching elements of the series converter 6 are individually connected to an output terminal Pout1 via another reactor Lout. Multiple capacitors C individually constituting the above-described reactors L and filters are connected in parallel between the input terminals and output terminals. When any abnormality is generated in the AC input voltage, a controlling device activates operation of the parallel converter 4 so that input current Ipara flowing through the parallel converter 4 can become output current Iout, thereby generating a state equal to the case in which the input side of the AC power supply source has been disconnected from the power supply switch. Concurrently, the controlling device activates operation of the series converter 6 so that the output voltage Vout can become the series-voltage commanding value Vout*.