To suppress the temperature rise of a secondary pickup coil without using a compulsive cooling means, by providing a secondary circuit with a coil consisting of a magnetic body for leading a magnetic flux generated by a primary circuit and a band-shaped conductor wound edgewise around this magnetic body.

A band-shaped conductor 13 for constituting a pickup coil 12 is wound on each back board 11d and 11e. For the band-shaped conductor 13, a film of insulating material is made on the surface of a conductor, and the end face on one side in the widthwise direction is abutted at a right angle against the face of each back board 11d and 11e of a pickup core 11, and at an angle, the band-shaped conductor 13 is turned up to change its direction by 90°. Likewise, the end face on one side is abutted at a right angle against the flank of each back board 11d and 11e, and such operation is repeated for every angle and flank of each back board 11d and 11e, whereby the edgewise winding of the band-shaped conductor 13 is performed. As a result, it becomes superior in heat radiating property by this edgewise winding, and this suppresses the temperature rise of the pickup coil.