There is provided with an antenna device, including: a conductive ground plane; first and second antenna elements connected to the conductive ground plane through first and second feeding points, respectively, directivities of the first and second antenna elements being arranged to be approximately perpendicular to each other; a phase adjusting circuit configured to provide a phase difference of approximately 180 degrees between first and second received signals by the first and second antenna elements, respectively; a combining circuit configured to combine the first and second received signals having a phase difference of approximately 180 degrees therebetween to obtain a combined signal; and a radio unit configured to process the combined signal.