To improve conversion efficiency of a switching power supply and reduce its noises, by regenerating to its input side the accumulated excitation energy of a transformer used in it, and by suppressing the ringing accompanied by its switching.

Proving in parallel with a switching field-effect transistor 22 for constituting a switching power supply a series circuit comprising a reset diode 23 and a capacitor 24, when turning off the transistor 22, the excitation energy of a transformer 21 is discharged via the reset diode 23 and charged in the capacitor 24. Thereafter, the charge in the capacitor 24 is regenerated via the primary winding of the transformer 21 to the input side of the switching power supply by the backward recovery current of the reset diode 23.