A transformer being capable of reducing cross regulation even in a case where the load is unbalanced and a switched-mode power supply apparatus using the transformer are provided. A transformer T has a core; a primary winding provided in the core; at least two secondary windings provided in the core around a winding axis which is the same as a winding axis of the primary winding; and at least two auxiliary windings provided in the core around a winding axis which is the same as the winding axis of the primary winding; respectively neighboring the secondary windings; and connected in parallel to each other. A switched-mode power supply apparatus has the transformer T; a switching element connected to the primary winding of the transformer T; and a control circuit configured to control the switching element.