UNINTERRUPTIBLE POWER SUPPLY SYSTEM

An uninterruptible power supply system that includes a selection unit that selects, of a plurality of uninterruptible power supply devices, a necessary number of uninterruptible power supply devices to supply power to a load. Each of the uninterruptible power supply devices performs, when this device is selected, an operating action of supplying power to the load, when this device is not selected, a standby action of not supplying power to the load, and during the standby action, a diagnostic action of diagnosing whether or not a predetermined part of this device has a failure. The selection unit changes uninterruptible power supply devices to be selected, such that the plurality of uninterruptible power supply devices have an equal operating time.