When power failure of an AC power source occurs during an eco-mode in which AC power is supplied via a bypass switch, an uninterruptible power source is configured to change over to inverter power supply in which AC power is supplied from an inverter. When turning off the bypass switch during power failure, a power conversion controller is configured to (i) control the converter to invert the DC power output by the battery into AC power and output the AC power to a node, and (ii) control the inverter to invert the DC power output by the battery into AC power and output the AC power to an output terminal. The power conversion controller controls the converter and the inverter such that phase and magnitude of an AC voltage output to the node become equal to phase and magnitude of an AC voltage output to output terminal.