An antenna device includes a base plate, an opposing conductor plate, a short-circuiting portion, a power-feed point, and at least one second-frequency electric-field impeding element provided to the opposing conductor plate, the base plate, or between the opposing conductor plate and the base plate. The second-frequency electric-field impeding element impedes a propagation of an electric field being vertical to the base plate and heading from the short-circuiting portion toward an outer edge portion of the opposing conductor plate, is induced at a second frequency higher than the first frequency, does not impede a propagation of the electric field induced at the first frequency, and is disposed such that an electrical area of the opposing conductor plate for a signal at the second frequency is an area of a proper size to generate an electrostatic capacitance that undergoes parallel resonance with the inductance of the short-circuiting portion at the second frequency.