A tag unit comprises two RFID tags arranged nearly parallel with each other, each RFID tag including an antenna, tag transmission/reception section, and storage section. The tag unit stores different serial numbers in the respective storage sections. A tag unit reading/writing apparatus writes the different serial numbers and common data in the storage sections of the respective RFID tags, and retains the same serial numbers of the respective RFID tags in its storage section 18. The tag unit reading/writing apparatus judges validity of reading and reusability of the tag unit, when it successfully reads both of the serial numbers and the data stored in the storage sections of the respective RFID tags.