To provide an antenna unit which is easy in lowering height and cost, and capable of after mounting to wheel rim and efficiently radiating vertical polarized waves to a side wall of a tire.

The antenna unit 1, which is mounted to a wheel rim 21 as a transponder of a system monitoring pneumatic tire, is provided to an end of an air valve 22. The antenna 1 includes a circuit substrate 2 on which an electronic circuit 6 and a ground conductive layer 7 are provided, an antenna element 3 of an inverted-F antenna type made of a metal plate, a shield case 4 made of a metal plate conducted to the ground conductive layer 7 and shielding the electronic circuit 6, and a resin case 5 installing those members. One side face along a longitudinal direction of a space sandwiched between the circuit substrate 2 and a radiation conductor 3a of the antenna element 3 is opposed to one side wall portion 20c of a tire 20, and the other side face is opposed to a wall face 21b of a wheel rim 21 having a distance corresponding to about a quarter of a wavelength of used waves.