In the case where there coexist general loads requiring backup only for a short duration and a specific load requiring backup for a longer duration, uninterruptible power with higher reliability is supplied to the specific load with the adoption of a simple configuration. A DC backup power supply system comprises an AC/DC converter for normally supplying DC power to loads in while (the general loads and the specific load, included), and a control circuit for supplying DC output backing up the loads in whole for relatively short time only from a battery at the time of a power outage, while supplying DC output backing up the specific load, such as a cache memory and so forth, for relatively long time. Thus, it is possible to implement an uninterruptible power supply system small in size and low in cost, provided with a function for proper use depending on a prevailing situation.