PURPOSE: To prevent a communication error owing to an interference of carrier waves having the same frequency in a moving body discriminating system furnishing plural interrogators.

CONSTITUTION: Interrogators X1, X2, X3... are provided in respective lanes. An oscillator 10 common to all the interrogators X1... is provided, its standard signal S6 is input to the PLL circuits of the interrogators, it is divided at a set specific ratio, and a carrier wave S7 at a specific frequency f1 is formed. The amplitude of the carrier wave S7 is diverted and modulated (ASK) in a time period A, while a nonmodulated carrier wave of the frequency f1 is output in a time period B, and a responder modulates this carrier wave, and returns it. On the other hand, in the interrogator X3, the perfectly the same frequency of carrier wave in the interrogator X1 is used through the PLLs, and the transmitting conditions in the time periods A and B are the same as in the interrogator X1. Since the carrier wave frequencies are coincident perfectly, a beat communication error owing to the mixture of the carrier waves in the interrogator X1 and in the interrogator X3 is never generated.