To provide a transformer which is equipped with separate windings that have a high inter-winding withstand voltage and a low inter-winding capacitance so as to have a very low and controlled inductance leakage and a high coupling coefficient.

The transformer whose inter-winding coupling coefficient and leakage inductance are set variable, or especially a high-frequency switching converter such as a high-frequency power converter which is formed by using the transformer is utilized. The transformer is equipped with an electromagnetic coupler which contains a magnetic medium that provides a magnetic flux path, two or more windings which are provided along the magnetic flux path so as to surround the magnetic flux path at a position apart from it, and a conductive medium arranged adjacent to the electromagnetic coupler.