An RFID tag reader/writer having a high degree of stability of communication with only a desired RFID tag. The frequency of a carrier wave to be transmitted from the RFID tag reader/writer is set on the basis of a resonance frequency of an antenna of the RFID tag located within a predetermined nearby communication area, which resonance frequency changes due to mutual coupling between the antenna of the RFID tag and an antenna of the RFID tag reader/writer. According to this setting, the RFID tag located within the nearby communication area has a high degree of sensitivity, and the RFID tag located outside the nearby communication area has a low degree of sensitivity, so that it is possible to effectively prevent an interference between the communication of the RFID tag reader/writer with the selected RFID tag and the communication of the reader/writer with the other or non-selected RFID tag.