In a switching power supply circuit including a primary side rectifying and smoothing circuit, a switching element, a converter transformer, a secondary side rectifying and smoothing circuit, and a switching element controlling section, a first primary side series resonant circuit is formed by a choke coil and a primary side series resonant capacitor, a second primary side series resonant circuit is formed by a leakage inductance occurring in a primary winding and the primary side series resonant capacitor, and a secondary side series resonant circuit is formed by a leakage inductance occurring in a secondary winding and a secondary side series resonant capacitor.