PURPOSE: To make it possible to confirm the operation of output protective function at the start of DC power supply by feeding DC output after confirmation that overvoltage detection and undervoltage detection are performed normally.

CONSTITUTION: DC output from a smoothing circuit 5 is fed to an output monitoring circuit 6 and a photocoupler PC0 is turned ON when an undervoltage detection level is exceeded. Upon turn ON of a thyristor Th, a photocoupler PC1 functions to confirm the operation of the thyristor Th. Upon turn ON of the photocoupler PC1 in an output protection control circuit 7, the photocoupler PC1 in a protective operation confirming/starting circuit 10 is turned ON and a relay A functions. Relay B functions through contact a2 of the relay A and relay TST is recovered through contact b2. Upon recovery of the relay TST, contact tst1 thereof opens to recover a relay P. Consequently, the photocoupler PC1, photothyristors PTh and the thyristor Th in the output protection control circuit 7 are entirely turned OFF.