The present invention relates to an array antenna for receiving a signal in a microwave band. The array antenna includes a plurality radiating element arranged in subarrays, each subarray having a first feeder connected to the radiating elements and a low noise amplifier (LNA) connected to receive the output of the first feeder, a frequency converter, and a second feeder for combining the outputs of the sub arrays from the LNAs. According to this structure, in the formula representing the noise temperature observed at the radiating elements, the gain the LNA appears in the denominator in the term representing the loss of the second feeder which combines the outputs of the sub arrays. Therefore, if the gain of the LNA is sufficiently large, it is possible to disregard the influence of the loss of the first and second feeders for combining the outputs of the sub arrays on the noise temperature. If each sub array is composed of a plurality of radiating elements and a first feeder for combining the outputs of the radiating elements, the loss of the second feeder is reduced more as the number of sub arrays is increased. As a result, the gain over temperature (G/T) is enhanced. Since it is possible to set the optimum number of sub arrays, it is possible to realize the enhancement of the G/T at the minimum necessary cost.