each of the antennas the area s is a product of the sum of thicknesses heights of the conductor layers 36 and the length of the counter electrodes 7 which are formed at a right angle while extending from the opening end of the split portion 5 toward the opening 2 in this example the electrostatic capacitance between the conductor layers 36 of different layers in the counter electrodes 7 is negligibly small as compared with the electrostatic capacitance between the conductor layers 36 of the same layers facing each other while sandwiching the split portion 5 the sum of the heights of the conductor layers of the counter electrodes 7 is for instance 0 8 mm when the conductor layers are six layers in total further the lengths of the counter electrodes 7 of the sr resonator antennas 10 and 20 are respectively 3 5 mm and 2 5 mm in this order for instance however the aforementioned sizes are merely an example and the present invention is not limited to the aforementioned sizes the number of the conductor layers 36 constituting the counter electrodes 7 sandwiching the split portion 5 of each of the antennas is not limited to six but may be any other number as far as it is possible to obtain a predetermined electrostatic capacitance between the counter electrodes.