A technology for detecting an RFID by a reader writer and transmitting harmonics for reading and writing the RFID timely without using an object detection sensor is provided. Using a nonlinearity of a rectifier or a demodulator in an IC chip of the RFID, a continuous wave or a modulated wave of two or more different frequencies is output from the reader writer. The RFID receives the continuous wave or the modulated wave of two or more different frequencies and the reader writer receives harmonics intermodulation generated by the rectifier or the demodulator in the IC chip. Therefore, the RFID can be detected without any special object detection sensor, and a modulated wave for reading or writing the RFID can be transmitted.