An antenna device includes a first conductor plate including a first end portion and a second end portion, the first conductor plate being provided with a first feeding portion between the first end portion and the second end portion, a second conductor plate including a third end portion connected to the first feeding portion, a fourth end portion located at a position away from the first conductor plate, and a plate surface of which width in a direction parallel to the first conductor plate increases with a distance from the third end portion toward the fourth end portion, and a third conductor plate including a fifth end portion capacitively coupling with the fourth end portion, a sixth end portion connected, on a same side as the first end portion with respect to the first feeding portion, to the first conductor plate, and a counter portion opposite the plate surface.