An antenna device comprising a rectangular plate-like soft magnetic body provided with a loop-like coil, wherein: the coil is formed of a conductive pattern and formed on an insulation sheet, the insulation sheet being disposed on the soft magnetic body as to have a part located on a front surface of the soft magnetic body and another part located on a rear surface of the soft magnetic body; the coil is disposed above the front surface and the rear surface of the soft magnetic body as to extend from the front surface toward the rear surface, so that a region surrounded by the coil on the insulation sheet overlaps with, at least in part, a specific edge, which is one of four edges of the soft magnetic body, and that the region surrounded by the coil does not include an opposite edge with respect to the specific edge; the coil of the conductive pattern is configured so that an area, which is in the front surface of the soft magnetic body and included in the region surrounded by the coil, is larger than another area, which is in the rear surface of the soft magnetic body and included in the region surrounded by the coil; characterized in that the coil located on the front surface of the soft magnetic body is configured to extend until at least the opposite edge as to have an area as large as possible in the front surface of the soft magnetic body so that a wide directivity between a direction perpendicular to the front surface of the soft magnetic body and another direction perpendicular to the specific edge is obtained; and the insulation sheet is configured to have both the part located on the front surface of the soft magnetic body and the other part located on the rear surface of the soft magnetic body extending until at least the opposite edge, the part located on the front surface having a same size as that of the other part located on the rear surface. the coil is formed of a conductive pattern and formed on an insulation sheet, the insulation sheet being disposed on the soft magnetic body as to have a part located on a front surface of the soft magnetic body and another part located on a rear surface of the soft magnetic body; the coil is disposed above the front surface and the rear surface of the soft magnetic body as to extend from the front surface toward the rear surface, so that a region surrounded by the coil on the insulation sheet overlaps with, at least in part, a specific edge, which is one of four edges of the soft magnetic body, and that the region surrounded by the coil does not include an opposite edge with respect to the specific edge; the coil of the conductive pattern is configured so that an area, which is in the front surface of the soft magnetic body and included in the region surrounded by the coil, is larger than another area, which is in the rear surface of the soft magnetic body and included in the region surrounded by the coil; the coil located on the front surface of the soft magnetic body is configured to extend until at least the opposite edge as to have an area as large as possible in the front surface of the soft magnetic body so that a wide directivity between a direction perpendicular to the front surface of the soft magnetic body and another direction perpendicular to the specific edge is obtained; and the insulation sheet is configured to have both the part located on the front surface of the soft magnetic body and the other part located on the rear surface of the soft magnetic body extending until at least the opposite edge, the part located on the front surface having a same size as that of the other part located on the rear surface.