An antenna device comprising: a grounded wiring board; a plate-shaped antenna element arranged to oppose the wiring board, with a gap formed between the antenna element and the wiring board, a surface of the antenna element opposing the wiring board having a regular polygonal shape; a plurality of grounding leg parts arranged at end parts of the antenna element and having respective extending tips connected to the wiring board; and a feeding leg part arranged at an end part of the antenna element, wherein the end part of the antenna element where the feeding leg part is arranged is one of vertices of the regular polygonal shape, the feeding leg part having an extending tip that extends from the end part, the extending tip being configured to connect to a transmission circuit or a reception circuit, and wherein the end parts of the antenna element where the plurality of grounding leg parts are arranged are the remaining vertices of the regular polygonal shape, characterised by a plurality of cutout parts respectively having a rectangular cutout shape that is formed by cutting out at least part of each outer side part of the antenna element, wherein the cutout shape of each of the plurality of cutout parts has one side with a length from a center point of each of the plurality of sides of the regular polygonal shape to a position not reaching each of the plurality of vertices of the regular polygonal shape in a direction towards each of the plurality of vertices, and wherein the plurality of grounding leg parts and the feeding leg part respectively have a widened leg part extending from the remaining portion of each outer side part of the antenna element that is not cut out. a grounded wiring board; a plate-shaped antenna element arranged to oppose the wiring board, with a gap formed between the antenna element and the wiring board, a surface of the antenna element opposing the wiring board having a regular polygonal shape; a plurality of grounding leg parts arranged at end parts of the antenna element and having respective extending tips connected to the wiring board; and a feeding leg part arranged at an end part of the antenna element, wherein the end part of the antenna element where the feeding leg part is arranged is one of vertices of the regular polygonal shape, the feeding leg part having an extending tip that extends from the end part, the extending tip being configured to connect to a transmission circuit or a reception circuit, and wherein the end parts of the antenna element where the plurality of grounding leg parts are arranged are the remaining vertices of the regular polygonal shape, characterised by a plurality of cutout parts respectively having a rectangular cutout shape that is formed by cutting out at least part of each outer side part of the antenna element, wherein the cutout shape of each of the plurality of cutout parts has one side with a length from a center point of each of the plurality of sides of the regular polygonal shape to a position not reaching each of the plurality of vertices of the regular polygonal shape in a direction towards each of the plurality of vertices, and wherein the plurality of grounding leg parts and the feeding leg part respectively have a widened leg part extending from the remaining portion of each outer side part of the antenna element that is not cut out.