The present invention may provide a switching power supply circuit for converting primary side direct-current voltage to secondary side direct-current voltage. The switching power supply circuit may include a choke coil, a converter transformer; a switching element, a primary side series resonant capacitor, a primary side parallel resonant capacitor, an oscillating and driving circuit, a secondary side rectifier circuit, and a control circuit. Resonance frequency of the primary side first series resonant circuit may be set at a frequency substantially twice resonance frequency of the primary side second series resonant circuit. Resonance frequency of the primary side parallel resonant circuit may be set at a frequency substantially equal to or higher than 1.5 times the resonance frequency of the primary side first series resonant circuit.