A switching power supply device includes a transformer, a switching unit which is connected with a primary winding of the transformer and configured to switch a current flowing to the primary winding, a start unit configured to start the switching unit, a voltage drop unit configured to lower output voltage from a secondary winding of the transform, and a current control unit configured to control an amount of a current flowing in the start unit when the switching unit is in an off state by lowering output voltage by the voltage drop unit.