An antenna device includes an antenna module configured to receive radio waves. A plate has a plurality of first through holes. A cover is coupled with the plate to define a space accommodating the antenna module. The cover includes a rib and a side wall surrounding the antenna module. The side wall has an end face opposing the plate and formed with a plurality of threaded holes. The rib is projected from the end face. A gasket has a part formed with a plurality of openings and is disposed between the rib and the plate. A plurality of screws are respectively screwed into the threaded holes through the first through holes and the openings. At least a part of each of the screws is surrounded by an associated one of the openings.