PURPOSE: To measure a characteristic of a resonator with high accuracy by detecting an electromagnetic wave outputted in the resonator direction from a signal output means by a signal detecting means, and finding respective mutual inductances between the output means, the resonator and a detecting means.

CONSTITUTION: A signal circuit 2 is composed of a signal source coil 2a and a sine wave signal generator 2b, and a detecting circuit 3 is composed of a detecting coil 3a and an AC voltmeter 3b. A tag 10 is composed of a coil 10a and a capacitor 10c. Now, the coils 2a and 3a and the central axis of the tag 10 are made to coincide with each other, and they are magnetically coupled with each other so as to become extremely sparse, and these inductances are respectively set as inductance L1 of the coil 2a, inductance L2 of the coil 3a and inductance L of the coil 10a, and L1 and L2 ≥L is realized. An electromagnetic wave outputted in the tag 10 direction by the circuit 2 is detected by the circuit 3, and a frequency characteristic of the tag 10 can be measured with high accuracy by the circuit 3 according to the respective mutual inductances between the circuit 2 and the tag 10, between the tag 10 and the circuit 3 and between the circuit 2 and the circuit 3.