In a switching power supply apparatus including an output transformer having a primary winding, a secondary winding and a feedback winding, a DC power source supplying DC voltage to the primary winding of the output transformer, an oscillation switching element having one terminal connected to the primary winding of the output transformer and a control terminal connected to the feedback winding, and a rectifying and smoothing circuit connected to the secondary winding of the output transformer, the switching power supply apparatus is characterized by further comprising a control circuit for flowing current through the secondary winding of the output transformer to one direction and a direction opposite thereto during a predetermined period after a time point where voltage is generated in the secondary winding of the output transformer.