An uninterruptible power supply is having a battery, a power inverter circuit for inverting the electrical power of the battery, and a voltage control circuit. The voltage control circuit controls the power inverter circuit, so that the output voltage of the power inverter circuit is maintained at a standby voltage lower than a rated voltage under the condition of a power failure detection signal being not output from an external power supply and the output voltage of the power inverter circuit equals the rated voltage under the condition of the power failure detection signal being output from the external power supply.