An antenna device includes an antenna coil, a magnetic core, and a metal member. The antenna coil is mounted on a flexible base. The antenna coil is wound in a loop shape or a spiral shape such that its winding center section is a coil opening section. The magnetic core passes through a slit opening section in the flexible base. The metal member has a rectangular opening. The antenna coil is exposed through the opening in the metal member. A front-side coil conductor is nearer the opening than the magnetic core, and thus a magnetic flux links the front-side coil conductor.