PURPOSE: To detect that a fuse is not inserted in a power unit yet, in the output monitor circuits of a plurality of power units which are connected in parallel to supply the load with the output voltage.

CONSTITUTION: A plurality of power units are supplied with power through fuses 3 from DC power sources 2 each for exclusive use, and the power is converted into desired voltage with a voltage converting circuit 4. The output voltage lines are connected in parallel and the voltages are supplied to the common load. Each power unit 1 is equipped with an exclusive output monitor circuit 5, and monitors input voltage and output voltage with an input monitor part 7 and an output monitor part 6 built in, and if it detects on abnormality, an alarm output part 8 gives an alarm and informs it. Hereby, the forgetting of the insertion of the fuse 3 is found out surely, and the drop of the output voltage by it is detected easily, and the adverse effect on the load is prevented.