An RFID tag device includes an RFID tag having a directivity of maximum gain of electromagnetic waves therefrom in a specific direction and an RFID tag-supporting base supporting the RFID tag. The RFID tag-supporting base includes a first surface on which the RFID tag is attached and a second surface which is oppositely located with and is positioned not in parallel to the first surface to orient the maximum gain direction of the electromagnetic waves from the RFID tag toward a desired direction.