UNINTERRUPTIBLE POWER SUPPLY DEVICE

A bidirectional chopper selectively performs: a charging operation to store DC power received from a DC bus in a power storage device; and a discharging operation to output DC power of the power storage device to the DC bus. An inverter converts the DC power received from the DC bus into AC power and supplies the AC power to a load, and converts regenerative power generated by the load into DC power and outputs the DC power to the DC bus. An SOC reference value is set for the power storage device, the SOC reference value being smaller than an upper limit value of a usable range of the SOC and larger than a lower limit value of the usable range. When an AC power supply is sound, a control device controls the bidirectional chopper such that the SOC of the power storage device reaches the SOC reference value.