a first electromagnetic contactor having a first terminal receiving a first AC voltage supplied from a first AC power supply and a second terminal connected to a load;

a semiconductor switch connected in parallel with the first electromagnetic contactor;

a rectifier that converts a second AC voltage supplied from a second AC power supply to a DC voltage;

a capacitor that smooths the DC voltage output from the rectifier; an inverter that converts a terminal-to-terminal voltage of the capacitor to a third AC voltage;

a second electromagnetic contactor having a first terminal receiving the third AC voltage and a second terminal connected to the load; and

a controller that controls the uninterruptible power supply apparatus, wherein

(i) in a first mode in which the first AC voltage is supplied to the load, the controller turns on the first electromagnetic contactor and turns off the semiconductor switch and the second electromagnetic contactor,

(ii) in a second mode in which the third AC voltage is supplied to the load, the controller turns off the first electromagnetic contactor and the semiconductor switch and turns on the second electromagnetic contactor, and

(iii) in a switching period in which the first mode is switched to the second mode, the controller turns on the semiconductor switch and the second electromagnetic contactor, turns off the first electromagnetic contactor, activates the inverter, and thereafter turns off the semiconductor switch, wherein the uninterruptible power supply apparatus further includes

a first current detector that detects AC current flowing between the second AC power supply and the rectifier; and

a second current detector that detects load current,

wherein the controller controls the rectifier based on result of detection by the first current detector and the second current detector, and passes AC current from the second AC power supply to the rectifier such that the terminal-to-terminal voltage of the capacitor becomes a reference voltage, wherein the AC current includes a feedback component having a value corresponding to a deviation between the reference voltage and the terminal-to-terminal voltage of the capacitor, and a feedforward component obtained by multiplying the load current by a gain.