An antenna device includes a substrate, a first antenna element extending in a direction perpendicular to a first surface of the substrate and functioning as a monopole antenna, a second antenna element provided adjacent to the first antenna element, extending in the direction perpendicular to the first surface of the substrate, and functioning as a monopole antenna, a ground layer provided in or on the substrate, a connection wire provided in or on the substrate and connecting the first antenna element and the second antenna element to each other, a power feeding line provided in or on the substrate and connected to the connection wire, and a first reflector provided in a direction in which the first antenna element and the second antenna element are adjacent to each other and facing the first antenna element and the second antenna element.