PURPOSE: To provide a switching system DC power supply in which a stable output can be obtained even at an abrupt change in load, and furthermore, in short-circuit state, an overcurrent can be prevented by responding instantaneously, by limiting the current periodically while circuit stability is increased.

CONSTITUTION: A current in an emitter of a switching transistor Q1 is detected by a current detection circuit 10, and according to the detected result, a control pulse is generated from a PWM circuit 11 to the switching transistor Q1 to control the switching transistor Q1.