PURPOSE: To control the number of uninterruptible power supplies to be operated under light load by providing a current transformer for detecting the output current built-in the uninterruptible power supply, a total current transformer for totalizing the secondary outputs from current transformers, and a relay for controlling the number of uninterruptible power supply to be operated.

CONSTITUTION: It is judged whether an uninterruptible power supply 2 is operating or not based on the operation of an operating condition detecting circuit 7. A judging circuit 8 then judges the appropriate number of the uninterruptible power supplies to be operated for current load based on the output current from a total current transformer 6, and a control object machine selecting circuit determines which uninterruptible power supply should be operated or stopped based on the individual judgement results. A control signal 10 is then provided to each uninterruptible power supply 2.