An antenna device configured for being installed in a positional relationship with respect to a cover (18; 68), comprising: one or more substrates (11; 61); a first radiating element (12; 62) disposed on a first region of a first surface of the one or more substrates (11; 61) that faces the cover (18; 68); and a second radiating element (15; 65) disposed on a second region of the second surface of the one or more substrates (11; 61) that faces the cover; wherein the first region and the second region are regions that do not overlap each other on the first surface and the second surface of the one or more substrates (11; 61) that face the cover (18; 68), wherein the antenna device further comprises: a first reflecting plate (13 63) adapted to reflect a portion of an electromagnetic wave radiated from the first radiating element (12; 62); and a second reflecting plate (16; 66) separate from the first reflecting plate (13; 63) and adapted to reflect a portion of an electromagnetic wave radiated from the second radiating element (15; 65); wherein the first and second radiating elements (12, 15; 62, 65) are disposed on a plane which is substantially parallel to the first surface and the second surface of the one or more substrates (11; 61) that faces the cover (18; 68), and wherein the first reflecting plate (13; 63) and the second reflecting plate (16; 66) are arranged parallel to the plane and disposed on a side opposite to the cover across the first antenna element and the second antenna element, respectively, and take different positions in a direction perpendicular to the first surface and the second surface of the one or more substrates (11; 61) that face the cover (18; 68). one or more substrates (11; 61); a first radiating element (12; 62) disposed on a first region of a first surface of the one or more substrates (11; 61) that faces the cover (18; 68); and a second radiating element (15; 65) disposed on a second region of the second surface of the one or more substrates (11; 61) that faces the cover; wherein the first region and the second region are regions that do not overlap each other on the first surface and the second surface of the one or more substrates (11; 61) that face the cover (18; 68), wherein the antenna device further comprises: a first reflecting plate (13 63) adapted to reflect a portion of an electromagnetic wave radiated from the first radiating element (12; 62); and a second reflecting plate (16; 66) separate from the first reflecting plate (13; 63) and adapted to reflect a portion of an electromagnetic wave radiated from the second radiating element (15; 65); wherein the first and second radiating elements (12, 15; 62, 65) are disposed on a plane which is substantially parallel to the first surface and the second surface of the one or more substrates (11; 61) that faces the cover (18; 68), and a first reflecting plate (13 63) adapted to reflect a portion of an electromagnetic wave radiated from the first radiating element (12; 62); and a second reflecting plate (16; 66) separate from the first reflecting plate (13; 63) and adapted to reflect a portion of an electromagnetic wave radiated from the second radiating element (15; 65); wherein the first and second radiating elements (12, 15; 62, 65) are disposed on a plane which is substantially parallel to the first surface and the second surface of the one or more substrates (11; 61) that faces the cover (18; 68), and wherein the first reflecting plate (13; 63) and the second reflecting plate (16; 66) are arranged parallel to the plane and disposed on a side opposite to the cover across the first antenna element and the second antenna element, respectively, and take different positions in a direction perpendicular to the first surface and the second surface of the one or more substrates (11; 61) that face the cover (18; 68).