An antenna device includes a dielectric substrate, a ground plane, a pair of antenna elements, a feeding section, and a pair of transmission lines. The ground plane is formed on a surface of the dielectric substrate. The antenna elements are flat, have different resonant frequencies, are formed on another surface of the dielectric substrate, and respectively have ends electrically connected to the ground plane. The feeding section feeds power to each of the antenna elements. The transmission lines carry out impedance conversion such that parts of the transmission lines which are connected to the antenna elements have impedances matching input impedances of the antenna elements, respectively, and such that part of the feeding section which is fed with the power has an impedance matching an impedance of the feeding section.