PURPOSE: To reduce the manufacturing cost of a transponder by transmitting a response signal for identifying a transponder repeatedly in receiving an interrogation signal thereby enhancing the receivability of an interrogator.

CONSTITUTION: A transponder is formed of a printed circuit and a receiving antenna 30 and a transmitting antenna 32 are subjected to orthogonal polarization. The antenna 30 stores a part of energy of an interrogation signal in a capacitor C through a diode 34. When the capacitor C is fully charged, a code generator 36 is actuated. The generator 36 is previously programmed with special codes transmitted three times and the output signal therefrom is delivered through a flip-flop 38 to a modulator 40 so that a part of energy received by the antenna 30 is directed again to the antenna 32. The flip-flop 38 is controlled by a logic circuit 42. Since the modulator 40 uses an interrogation signal received as a transmission source for an output carrier signal, a precise frequency component, e.g. an active power transmitter, is not required.