A vehicle-mounted antenna system according to the present invention comprises: a first antenna system in which a plurality of first antenna elements disposed in a structure mounted on a vehicle perform multiple-input and multiple-output (MIMO); and a second antenna system in which a plurality of second antenna elements attached to a side surface of a polyhedron disposed in the structure perform beamforming, wherein the disposed plurality of first antenna elements are formed, in the front and back of the polyhedron, on a substrate erected at a predetermined angle to the structure, and thus, the present invention may provide an antenna array optimized in terms of spacing between antenna elements in a flat-type vehicle antenna provided with an LTE antenna system and a 5G antenna system.