To improve safety and reliability and to allow miniaturization by winding, in normal winding, a secondary coil in the direction wherein the identical magnetic flux is generated at a magnetic core through a bobbin, and winding such a primary coil as formed with a thin plate like conductor around the magnetic core.

U-type and L-type cores 1 are coupled, each with one pair, to form almost square core and at two facing side legs of the square core, a bobbin 2 wherein, comprising a hole 3 with flanges 4 on both ends, a coil is wound in normal winding manner is attached. The normal winding is generally a single layer, however, for a secondary coil 6 of larger current capacity, it is stacked in two stages, and one side of first and second stages is connected to a low voltage terminal, while the other side to a high voltage terminal. Likewise, further multi-stage is provided. At such magnetic core part as clamped between two secondary coils 6, are primary coil 8 formed of a thin plate like conductor is wound.