PURPOSE: To reduce the interruption of reception and detect a receiving code in an interrogator precisely by extending the period of status change for the timing to be given to the transmission wave of the interrogator and narrowing transmitting band width.

CONSTITUTION: A microwave generated in the transmitting part 1 of an interrogator set up on the ground is continuously penetrated into the receiving antenna 11 of a responsor 102 and supplied to each part of the responsor 102 as DC power through a rectifier 12. A code sending circuit 13 reads out the stored contents from a memory unit 14 and correspondingly switching circuit 16 FSK-modulates the oscillation frequency of a modulated wave oscillator 15. The status of the transmitting wave of the interrogator 101 can be changed periodically so as to be detected in the timing detector 20 of the responsor 102 in order to give the timing for the code sending start of the responsor 102.