An antenna device includes a ground plate that is a plate-shaped conductor member, a patch portion that is a plate-shaped conductor member placed to be opposed to the ground plate with a predetermined distance therebetween, a short-circuit portion that is a conductor member electrically connecting together the patch portion and the ground plate, and an additional conductor that is a plate-shaped conductor member placed to be opposed to the patch portion with a predetermined distance therebetween. The additional conductor is placed on a side of the patch portion on which the ground plate is not placed. An inductance of the short-circuit portion, a capacitance formed by the ground plate and the patch portion, and a capacitance formed by the patch portion and the additional conductor are used to perform parallel resonance.