A switching power supply apparatus has a line-to-ground capacitance C31 between a first winding terminal of a transformer and a conductor portion, and a line-to-ground capacitance C32 between a second winding terminal of the transformer and the conductor portion. The switching power supply apparatus is provided with a capacitor being connected between a first output terminal of a switching circuit and the first winding terminal of the transformer, and a capacitor being connected between a second output terminal of the switching circuit and the second winding terminal of the transformer. Capacitances of the capacitors C21, C22 are set to satisfy: C21>C22, for C31>C32; C21=C22, for C31=C32; and C21