PURPOSE: To provide a highly reliable and economical uninterruptible power supply system wherein damage to a d.c. power supply, an output circuit and/or a load circuit and failure of a load by erroneous connection between the output circuit and the load circuit.

CONSTITUTION: An uninterruptible power supply system consists of a d.c. power supply 100; a load circuit 200 composed of a storage battery 50 and a load 60; and an output circuit 1000 that connects the d.c. power supply 100 and the load circuit 200. The output circuit 1000 includes a semiconductor switch (600, 700), connected with the load circuit 200 and having reverse breakdown voltage, and an output detecting circuit 800 that detects the output voltage of the output circuit 1000. The semiconductor switch is closed according to the signals from the output detecting circuit 800.