A communication system includes multiple servers each functioning as an active system or a standby system and a relay device that relays communication between a server of the active system and one or more client terminals operated by clients. Each of the servers includes a priority level determining unit that determines a priority level of each of the clients, a virtual port creating unit that creates, in the server, a virtual port corresponding to a virtual LAN assigned to each of the client terminals, and a communication processing unit (CPU) that instructs the relay device to change a destination of communication performed between the client terminal and the server to the virtual port. The CPU gives an instruction of change of the destination of the communication in order of highest to lowest priority level of the client when a malfunction occurs in another server of the active system.