An uninterruptible power supply is able to switch an operation mode between an on-line operation method and an off-line operation method for a wider range of commercial power supplies. An uninterruptible power supply 1 includes; a rectifier 11 for converting AC power supplied from a commercial power source 2 into DC power; a storage battery 12 for storing the DC power converted; an inverter 13 for converting the DC power into AC power; a selecting means (14 to 18) for selecting one of an off-line operation mode and an on-line operation mode according to a detection result of detecting a condition of the AC power supplied from the power source 2; in the off-line operation mode, AC power supplied from the power source 2 is supplied to a load device 3 under normal operating condition, and when any failure happens in relation to the AC power supplied from the power source 2, the uninterruptible power supply supplies a power output generated by the inverter 13 to the load device 3; in the on-line operation method, a power output from the inverter 13 is supplied to the load device 3 all the time including a period of normal operating condition; and a setup means (19) for variably setting criterion values, with which the selecting means (14 to 18) selects an operation mode depending on the condition of the AC power supplied from the power source 2.