An uninterruptible power supply apparatus includes a converter (5) configured to convert AC power into DC power, and an inverter (10) configured to convert DC power into AC power and supply the converted power to a load (24). The load (24) is configured to receive an AC voltage within a range of allowable input voltage to consume constant AC power. The uninterruptible power supply apparatus has maximum efficiency η when a ratio of load capacity to rated capacity of the uninterruptible power supply apparatus is a predetermined value α. The uninterruptible power supply apparatus further includes a control device (18) configured to control an output voltage of the inverter (10) within the range of allowable input voltage so as to increase the efficiency η, when the ratio of the load capacity to the rated capacity is different from the predetermined value α.