An antenna system is effectively formed in a limited space, to make a configuration of the antenna system a simple mechanism. In a monopulse radar apparatus, an antenna unit includes an array antenna (a wide-beam array antenna) formed by a part of antenna elements that are constituent elements of the antenna unit (an area K1), and a plurality of narrow-beam array antennas formed as array antennas having a narrower beam width than the array antenna. A monopulse process is performed based on an output of a predetermined pair of array antennas (areas K2 and K3) from among the array antennas formed as the narrow-beam array antennas.