A communication system includes a communication device, a plurality of servers configured to communicate with the communication device, a plurality of communication lines corresponding to a plurality of servers, and a relay server. The relay server includes a communication acquisition part configured to acquire the device identification information for identifying the communication device, the communication content information representing the communication content of the communication device, and the line identification information for identifying a plurality of communication lines respectively; and a communication information output part configured to output the communication content information and the device identification information acquired by the communication acquisition part over a communication line corresponding to the line identification information acquired by the communication acquisition part. The communication device includes a communication output part configured to correlate and output the device identification information, the communication content information, and the line identification information to the relay server.