This patent specification describes an antenna device which includes a non-directional antenna having a radiating element and a ground plate, a coaxial line configured to feed an electromagnetic power to the non-directional antenna, a dielectric film arranged on the ground plate, including a dielectric material, a short circuit line arranged on the dielectric film, formed of a conductive pattern and configured to connect an inner conductor of the coaxial line to an outer conductor of the coaxial line and a switch arranged at a portion of the short circuit line to switch a state between a non-shorted state and a shorted state.