An uninterruptible power supply apparatus includes: an electromagnetic contactor including a first terminal that receives a first AC voltage supplied from an AC power supply and a second terminal connected to an AC node; a capacitor that is connected to a DC line and stores DC power; a converter that transmits and receives electric power between the AC node and the DC line; and a controller. In an activation mode, the controller turns on the electromagnetic contactor after controlling the converter such that a frequency and a phase of a second AC voltage supplied from the converter to the AC node match a frequency and a phase of the first AC voltage. In a normal operation mode after the activation mode, the controller controls the converter such that a DC voltage of the DC node attains to a reference voltage.