An antenna device includes a first triplate line and one or more second triplate lines that are connected to the first triplate line so as to cross the first triplate line. Two ground plates of each second triplate line include respective flange portions that are in contact with and fixed to a surface of one of two ground plates of the first triplate line. At least one of a surface of each flange portion that contacts the one of the ground plates of the first triplate line and the surface of the one of the ground plates of the first triplate line that contacts each flange portion is provided with an irregular portion for reducing a contact area between each flange portion and the one of the ground plates.