UNINTERRUPTIBLE POWER SUPPLY DEVICE

An uninterruptible power supply device has a normal mode of supplying a load with electric power supplied from an AC power supply, a backup mode of supplying the load with electric power stored in a storage battery during a power failure, and a deterioration diagnosis mode of diagnosing a deteriorated state of the storage battery. A controller periodically shifts to the deterioration diagnosis mode during execution of the normal mode. During execution of the deterioration diagnosis mode, the controller stops charging of the storage battery by stopping an operation of the power converter. The controller measures a voltage of the storage battery at a first timing after a lapse of a first time period from stopping charging of the storage battery, and diagnoses the deteriorated state of the storage battery based on a measured value of the voltage of the storage battery at the first timing.