PURPOSE: To supply a stable power to a load without deteriorating an efficiency by a method wherein the sub-output of a switching DC power supply apparatus having a plurality of outputs is synchronized with the main output with a simple structure.

CONSTITUTION: A DC voltage applied to the primary winding F1 of an insulating transformer (converter transformer) T1 is switched by a first switching device Q1 and pulse outputs are induced in secondary windings S1 and S2. The pulse output of the secondary winding S2 is switched by a second switching device Q2 for stabilization and smoothed and then supplied to a load. At that time, the output voltage of the secondary winding S2 is detected by a detector DT2 and, further, the pulse output of the secondary winding S1 is integrated by an integrating circuit IG1 to obtain a triangular wave signal. Then the detected value of the output voltage is compared with a triangular wave output by a comparator CMP2 provided as a PWM controller. In accordance with the comparison result, the continuity angle of the second switching device Q2 is controlled.