A switching power supply apparatus includes: an DC/AC converter unit for converting a DC voltage to an AC voltage based on switching operation of switching devices; a transformer for converting the AC voltage to an AC voltage having a voltage value; a resonance circuit provided between the DC/AC converter unit and the transformer; an AC/DC converter circuit for converting an AC voltage from the transformer to a DC; an output detector unit for detecting an output voltage or an output current of the apparatus; a duty ratio controller unit for controlling a duty ratio of switching of the apparatus such that a detected output voltage or current becomes a target value; an energy detector unit for detecting an energy accumulated in the resonance circuit; and a controller unit for controlling a switching frequency such that a detected energy becomes a threshold value.