It is detected of an interference of a reverse link communication in a feeder link of a communication relay apparatus, which tends to occur when the number of movable aerial-floating type communication relay apparatuses increases in the same area. A communication system comprises a plurality of movable aerial-floating type communication relay apparatuses that respectively include a relay communication station of performing a service-link radio communication with a terminal apparatus, plural gateway stations that respectively perform a feeder-link radio communication with the plurality of the communication relay apparatuses, and a common baseband processing apparatus connected to the plural gateway stations. The baseband processing apparatus detects an interference of the reverse link communication due to an interference wave from a relay communication station of another communication relay apparatus, the interference wave interfering with a signal wave from the relay communication station of the communication relay apparatus connected to the gateway station, based on plural reception signals received by the plural gateway stations.