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 10; a primary winding 11 provided in the core 10; at least two secondary windings 12, 13 provided in the core 10 around a winding axis which is the same as a winding axis of the primary winding 11; and at least two auxiliary windings 14, 16 provided in the core 10 around a winding axis which is the same as the winding axis of the primary winding 11; respectively neighboring the secondary windings 12, 13; 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 11 of the transformer T; and a control circuit configured to control the switching element.
