A transformer in this invention comprises substrates constructing a primary winding, substrates constructing the secondary winding, and a core member disposed around the substrates. The substrates defining the primary winding are provided with insertion holes for passing a middle leg portion of the core member, and patterned conductors having one turn. The transformer further comprises an interlayer connection member. The interlayer connection member is located inwardly of the patterned conductors of the substrates defining the primary winding. The interlayer connection member is located on the same side of the patterned conductors defining the primary winding. The interlayer connection member is configured to establish the electrical connection of the patterned conductors defining the primary winding.