A communication system (50) according to the present invention includes a first communication device (10) that functions as a main device, and a second communication device (20) that functions as a backup device for the first communication device (10). A second functional unit of the second communication device (20) transmits a response request to a first functional unit of the first communication device (10), and transmits a transfer-destination switching request to the first communication device (10) if a response to the response request has not been received from the first functional unit. If a first transfer unit receives the transfer-destination switching request from the second functional unit, the first transfer unit switches a transfer destination of packets that should be processed by the second functional unit from the first functional unit to the second functional unit.