PURPOSE: To completely prevent the generation of a virtual image by a reflecting body by stopping the generation of the high frequency query signal of an interrogator when an antenna is turned in the direction of a radio reflecting body.

CONSTITUTION: The azimuth A of an antenna to a radio reflecting body 3 is previously measured, and this angle is set into an azimuth setting part 14. A high frequency query signal is emitted from an interrogator 10 through an antenna 11. In the meantime, a comparing part 15 compares an azimuth signal from the antenna 11 with the set azimuth signal of the reflecting body 3, and a trigger signal is outputted to a challenge on/off part 16 only when the pointing direction of the antenna 11 is turned to the azimuth A of the reflecting body 3. The on/off part 16 outputs a challenge control signal to the interrogator 10 according to the trigger signal, and the interrogator 10 stops the generation of the high frequency query signal only at the time of the challenge control signal being inputted. The generation of a virtual image by the reflecting body can be thereby prevented positively.