A transformer that is capable of setting any characteristics of a detection voltage of a detection winding and accurately detecting an output voltage includes a bobbin, a magnetic core, a first input winding, an output winding, a second input winding, and a detection winding. The bobbin is tubular and includes a plurality of winding regions located at its outer portion. The magnetic core is inserted in the bobbin. The first input winding is wound in a first winding region. The output winding is wound in a second winding region adjacent to the first winding region. The second input winding is wound in a third winding region adjacent to the second winding region. The detection winding is wound in the vicinity of the first input winding. The first input winding and the second input winding have different numbers of turns and are connected in series in the same winding direction.