PURPOSE: To establish the coupling circuit suitable for mass-production, by placing the coupling plate between the primary and secondary tuning circuits and by making variable the opposing area and distance of the coupling plate so that the degree of coupling can be adjusted.

CONSTITUTION: The opposing primary coil 23 and secondary coil 24 are placed via the separating plate 22 separating about 1/2 of the internal of the console 21. The variable air capacitors 25 and 26 are connected to the coils 23 and 24 without separating plate, forming the tuning circuits 31, 32 for the primary and secondary. The coupling plates 27 and 28 are placed on the capacitors 25 and 26, and one end of the coupling 27 is slightly projected to the secondary capacitor 26, one end of the coupling plate 28 is projected to the primary capacitor 25 and setting is made so that the end of the coupling plate 27 is opposed to the plate 28 parallelly. The stray capacitance of the primary and secondary side is determined with the area and the distance of the opposing part of the coupling plates 27 and 28 and the coupling degree can be determined, then the degree of coupling can easily be adjusted and coupling circuit suitable for mass-production can be established