PURPOSE: To obtain a DC switching power supply which can feed power stably to a load even upon instantaneous interruption of input.

CONSTITUTION: Under normal state where input is not interrupted, input power is converted and fed to a predetermined load while at the same time AC output is rectified and a third capacitor 3 is charged. Under emergency state where the input is interrupted, power from a second capacitor 3 is interrupted by a switching element 12 and a generated AC output is converted through a third diode 13 into DC power which is fed to the load.