A transformer being capable of suppressing the output voltage difference, and a switched-mode power supply apparatus using the transformer are provided. A transformer T has a core 10; a primary winding 11 provided in the core 10; a gap 15 provided in the core 10 at a location where the primary winding 11 is provided; and at least two secondary windings 12, 13 provided in the core 10 and spaced apart from both sides of the primary winding 11 as well as the gap 15 at an equal distance in a winding axis direction of the primary winding 11. A switched-mode power supply apparatus has the transformer T; a switching element connected to the primary winding 11 of the transformer T; and a control circuit configured to control the switching element.
