A circularly polarized antenna has a dielectric substrate, a ground conductor piled up on one surface side of the dielectric substrate, a circularly polarized antenna element formed on an opposite surface of the dielectric substrate, a plurality of metal posts whose respective one end sides are connected to the ground conductor and penetrate the dielectric substrate along a thickness direction thereof, and whose respective other sides extend up to the opposite surface of the dielectric substrate, the plurality of metal posts configuring a cavity by being provided at predetermined intervals so as to surround the antenna element, and a conducting rim which short-circuits the respective other end sides of the plurality of metal posts along an array direction thereof, and is provided so as to extend by a predetermined distance in a direction of the antenna element at the side of the opposite surface of the dielectric substrate.