PURPOSE: To obtain a stable and highly efficient radio frequency magnetic field generating detector by a method wherein 2nd and 3rd variable capacitors are connected to the respective terminals of 1st variable capacitor to form a balanced impedance matching circuit and a balun is connected to the matching circuit.

CONSTITUTION: 2nd variable capacitor 8b and 3rd variable capacitor 8c are connected to the respective terminals of 1st variable capacitor 8a which provides a capacitive coupling between a pair of adjoining wings 1b to form a balanced impedance matching circuit 8 and a balun 9 is connected to the balanced impedance matching circuit 8. Then the resonance frequency and the impedance matching frequency of a microwave circuit which functions as an equivalent series resonance circuit are adjusted by the 1st variable capacitor 8a. As a result, voltages whose magnitudes are equal and polarities are opposite to each other are applied to electrodes 7a and 7b so that the balanced feeding of the LC series resonance circuit can be completely realized.