To obtain an array antenna system of a radial line feeder type, which has a mono-pulse comparator with a simple configuration and has a very small loss.

An array antenna 22 using radial lines 1, 7 energized through feeder probes 6a-6d is provided with a 2nd radial line 7 connected to the line 1 via the feeder probes 6a-6d, a sum signal probe 10 is placed to nearly the center position of the feeder probes and difference signal probes 9a, 9b are placed at a position where a difference of distances between a desired cut face of the array antenna 22 and two sets of feeder probe groups configured by dividing the feeder probes 6a-6d is nearly an odd number multiple of a half wavelength.