A compact antenna structure having substantially no directivity independent of a tag direction in an RFID tag is proposed. The antenna structure includes a pair of dipole antennas is provided in a cross dipole antenna to be applied to an RFID system for communicating information between a reader/writer and the tag using a high-frequency radio signal. The above pair of dipole antennas further includes lines extending from a feed point and mutually intersecting crosswise, and also triangularly expanded lines from the bent ends of the above lines. Each total length of the above pair of dipole antennas is longer than 2/λ of a use frequency λ.