The invention aims at putting a single ended resonance type converter capable of obtaining a ZVS operation over an overall corresponding load region to practical use. A switching power supply circuit includes a primary side switching converter of a class-E resonance type, and a choke coil of a primary side series resonance circuit of the class-E resonance type converter is used as a primary winding of an insulating converter transformer. With this structure, a choke coil winding to be inserted into an input line of a DC input voltage is arranged between a positive-polarity end portion of a smoothing capacitor, and a node between the primary winding and a primary side series resonance capacitor. Thus, a resonance pulse voltage generated in the choke coil winding is superimposed on an AC voltage in the primary winding. Also, a primary series resonance current is caused to separately flow through the choke coil winding.