UNINTERRUPTIBLE POWER SUPPLY DEVICE

An uninterruptible power supply device provided between a commercial power system and an essential load and supplying AC power to the essential load includes: a power supply part that has an energy storage device and is connected to a power line for supplying power from the commercial power system to the essential load; an open switch for opening/closing the power supply line, the open switch being provided on the power line on the commercial-power-system side of the power supply part; a system abnormality detection part for detecting a system abnormality occurring on the commercial-power-system side of the open switch; and a control part that, when the detected system abnormality is equal to or greater than the tolerance of the essential load or the power supply part against system abnormalities, opens the open switch and supplies AC power from the power supply part to the essential load.