A
transformer including a first
magnetic core and a second
magnetic core having the shape of a flat plate. In the first
magnetic core, a middle leg and substantially L-shaped outer legs whose inner sides are formed as circular-arc-shaped surfaces are provided in a standing manner at the central portion and at the comers of a rectangular flat plate, respectively. In addition, in the first magnetic core, a winding accommodating portion is formed between the circular-arc-shaped surfaces of the corresponding outer legs and an outer
peripheral surface of the middle leg. Three coil portions, formed by spiral winding operations in level states, are placed upon each other, and are inserted into the winding accommodating portion of the first magnetic core. Then, from above the coil portions, the second magnetic core is placed thereon in order to form an integral structure. At this time, at the side portions of the flat plate, an external line of the flat plate and an external line of the winding accommodating portion coincide with each other. Therefore, the
footprint of the
transformer becomes small. Accordingly, in the invention, the
transformer can be made smaller and thinner as a result of making the
dead space that exists during the mounting of the coil to the magnetic core as small as possible.