PURPOSE: To make it possible to mass-produce antenna unit of high performance by making it easy to attach and detach a matching element in test adjustment, by equipping the opening surface of a ridge waveguide with a dielectric plate and iris plate for impedance matching.

CONSTITUTION: Since iris plate 2 and dielectric plate 3 are fixed via screw holes 2b and 3a at ridge waveguide opening 1 as the element antenna of a phased array antenna, capacitive reactance provided by dielectric plate 3 through the operation of iris plate 2 serving as a pure conductive element, so that impedance matching with free space will become possible covering a broad band. Further, since the generation of a surface wave component propagating in the array opening direction is also suppressed, mutual coupling between element antennas is lessened and the fluctuations of active impedance are made extremely small.