Transmission and reception of the identification number to/from an interrogator includes an interrogator that reads a recognition number from a responder by radio. When a clock pulse is modulated on a high-frequency carrier and transmitted to the responder from the antenna of the interrogator, there are a first case when the clock pulse interval is short and a second case when the clock pulse interval is long. By combining the clock pulse of the first case and the clock pulse of the second case so as to control the read of the recognition number from the interrogator, it is possible to realize reduction of the semiconductor chip size of the responder and suppress the cost of the semiconductor chip.