An antenna having a construction enabling simple adjustment of impedance at the antenna connection terminal has a grounded plate; a plate-shape radiating conductor, positioned parallel to the grounded plate; a feed line conductor, one end of which is connected to a feed point of the plate-shape radiating conductor, the other end of which is connected, as an antenna terminal, to an inner conductor of a coaxial cable, and which is perpendicular to the plate-shape radiating conductor; and a conductor disc, electrically connected to the feed line conductor, and positioned parallel to the ground plate, the distance from the conductor disc to the grounded plate being adjustable.