PURPOSE: To make a responder function in two methods, by converting the responder to apply both to the case to use an unmodulated carrier wave from an interrogator, and the case to use a carrier wave generated by the responder.

CONSTITUTION: When an unmodulated carrier wave from an interrogator is used, the input impedance of an oscillator 25 is converted to be infinite, and the nonpass and the pass of a diode 23 are switched according to the level of the receiving signal. And in case of the nonpass, the received unmodulated wave is reflected at the input end of the diode 23 and transmitted to the space. And in case of the pass, the wave passes through the diode 23, it is reflected at the input end of the oscillator 25, and the modulated wave having the phase difference π is transmitted through a transmission passage 24 set at the wavelength λ. Consequently, the carrier wave is modulated into two conditions having the phase difference π of the carrier wave. And when the carrier wave generated by the responder is used, the oscillator 25 is made in the line impedance, the carrier wave output by the oscillator 25 is output to the diode 23, and the pass, the cutoff, and the amplitude modulation are applied to an antenna 21 according to the two values of modulation signals.