Abstract: A power converting apparatus used as an uninterruptible power supply connected between an AC power supply and a load includes first and second power converters, a power supply voltage sensor, an output voltage sensor and a controller. The controller calculates rms voltage values of positive and negative half-cycles of an AC output voltage detected by the output voltage sensor, generates a target voltage command based on calculation results and a given reference voltage command, and controls the first power converter based on an output voltage command which is equivalent to a voltage difference between the AC power supply voltage detected by the power supply voltage sensor and the target voltage command.