Abstract: A transmitting/receiving antenna radiates a sweep frequency signal as an interrogation signal to an answer device. The answer device includes a plurality of resonators which are connected in parallel and have different resonance frequencies and answer antenna means connected in parallel with these resonators and transmits resonance answer signals including ringing answer signals corresponding to the resonators, in response to the interrogation signal. A phase adjusting means is provided which receives at the inputs the sweep frequency signal and the resonance answer signal and adjusts the phases of these signals to have the phase difference therebetween 0.degree. or 180.degree.. The output of the phase adjusting means is applied to a detector where the ringing answer signals are detected to form binary answer signals corresponding to the ringing answer signals, respectively. An object with the answer device fixed thereat is identified from these binary answer signals.