To provide an antenna unit incorporated in a tire capable of performing a communication between a transponder in a tire and an external antenna at a car body side without trouble at all the time even when a vehicle travels or stops.

In the antenna unit incorporated in the tire, which is provided with a transponder 4 capable of communicating with an external antenna 7 at the car body side and is attached into the tire 1, one or more parasitic antennas 5 capable of being resonated at a frequency equal with that of an antenna element of the transponder 4 are disposed in the tire 1, so that radio waves radiated from the transponder 4 and propagated within the tire 1 can be radiated again by the parasitic antennas 5. When mounting a plurality of parasitic antennas 5, these parasitic antennas 5 are set along a circumferential direction of the tire 1 so as to be spaced from the transponder 4 approximately at equal intervals. Furthermore, one parasitic antenna 5 may also be attched to a position approximately in point-symmetry with the transponder 4 in the tire 1.