A transformer includes: a core assembly 1 composed of a pair of E-shaped cores 11, 12 each having two side leg portions 1a, 1b and a central leg portion 1c therebetween, end surfaces of the central leg portions 1c and end surfaces of the side leg portions 1a, 1b of the E-shaped cores 11, 12 oppose each other, respectively, and a gap G is provided between at least the end surfaces of the central leg portions 1c; a primary coil N1 formed by winding round wire around a perimeter of the central leg portion 1c; and a secondary coil N2 formed by winding rectangular wire around a perimeter of the central leg portion 1c by edgewise winding, wherein a space for reducing leakage flux from the gap G that acts on the secondary coil N2 is provided between the secondary coil N2 and the gap G.