An antenna device includes an antenna coil, a magnetic sheet, and a metallic member. The antenna coil is formed on a flexible base. The antenna coil is wound into a loop or a spiral in which a winding central portion is a coil opening portion. The magnetic sheet is disposed at a back surface of the flexible base. A square opening is formed in the metallic member. The antenna coil is exposed from the opening of the metallic member. A first side of the antenna coil is hidden by the metallic member and part of the coil opening portion and a second side are exposed from the opening, so that a magnetic flux links with the second side.