To provide a highly efficient power unit which is improved in efficiency and reduced in thickness, weight and cost by finding the spot at which the magnetic flux generated from the coil of a primary-side circuit can be transmitted efficiently to the coil of a secondary-side circuit by studying the relation of the distance between facing coils and the inside diameters of the coils.

This power unit is provided with a first coil which is loaded in a first enclosure and to which an electric current is supplied, and a second coil which is loaded in a second enclosure and to which electric power is transmitted from the first coil. The second coil is constituted of an air-core coil, and the inside diameter of the winding of the air-core coil is made larger than that of the winding of the first coil. In addition, the first and second coils are arranged so that the surfaces of the windings of the coils may be faced oppositely to each other.