To reduce the communication time between an interrogator and a responder.

An interrogator 11 sends an ID invitation signal. When the interrogator 11 receives a reply from a responder 12, the interrogator 11 decides a pseudo ID code whose data amount is smaller than that of the received ID code, adds the pseudo ID code and the ID code to a message and transmits the resulting message to the responder 12. When the received ID code is coincident with its own ID code, the responder 12 stores the pseudo ID code added to the ID code and transmits a reply command. The interrogator 11 discriminates whether or not the reply from the responder 12 is received, and when receiving the reply, the interrogator 11 stores the pseudo ID code corresponding to the ID code. Then the communication with the responder 12 is conducted by using the pseudo ID code.