A thin antenna includes an antenna element, a first spacer, a second spacer, a first ground plane and a second ground plane. The antenna element is formed in a column shape, and has a top surface and a bottom surface facing each other. The first and second spacers are made of an insulating material. The first ground plane is formed larger than the top surface of the antenna element. The second ground plane is formed larger than the bottom surface of the antenna element. The first ground plane is disposed to face the top surface of the antenna element via the first spacer. The second ground plane is disposed to face the bottom surface of the antenna element via the second spacer. A power is fed at one of the top surface and the bottom surface of the antenna element.