A switching power source apparatus has a series circuit connected to both ends of a DC power source Vdc1 and having a reactor L3, a primary winding 5a of a transformer T, and a switch Q1, a series circuit connected to both ends of the primary winding and having an auxiliary switch Q2 and a clamp capacitor C3, a saturable reactor SL1 connected in parallel with the primary winding, a rectifying-smoothing circuit D1, D2, L1, C4 for rectifying and smoothing a voltage generated on a secondary winding 5b, and a control circuit 10 for alternately turning on/off the switches Q1 and Q2 and turning off the switch Q2 if a current of the switch Q2 increases due to saturation of the saturable reactor SL1. The reactor L3 is made of leakage inductance between the primary and secondary windings. In the transformer T, the primary winding is divided into two sections that are connected in series, the secondary winding is arranged between the two sections of the primary winding, and the numbers of turns of the two sections of the primary winding are adjusted to adjust the leakage inductance to saturate the saturable reactor SL1.