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