A control device controls an uninterruptible power supply device such that a transition is made to an inverter power supply mode when a power source abnormality detecting unit detects a power failure of an AC power source in a case where a commercial power supply mode is selected. The power source abnormality detecting unit computes an estimated value of the maximum value of AC input voltage, based on detected values of the instantaneous value and the phase of AC input voltage supplied to an input terminal. The power source abnormality detecting unit is configured to store temporal transition of the maximum value when the AC power source undergoes a simulated power failure and determine a power failure of the AC power source, based on comparison between temporal transition of the estimated value of the maximum value and the stored temporal transition of the maximum value during power failure.