Provided is a communication system including a relay device that relays communication between a base station and a communication terminal, and a management server, the management server including a receiving unit that receives from each base station information about a communication terminal belonging to the base station and about the relay device, and a determination unit that determines, on the basis of the information received from each base station by the receiving unit, a relay device that is performing communication interfering with communication in an adjacent cell. When the relay device is determined as a relay device that is performing communication interfering with the communication in the adjacent cell, the relay device determines an interference avoidance control and executes the determined interference avoidance control.