A server and a client hold common secret information in respective secret information holding units. A server Cyclic Redundancy Check (CRC) unit in the server generates a CRC code after adding the secret information to communication data, and transmits the communication data with the CRC code attached. A client CRC unit of the client generates a CRC code after adding the secret information to communication data, and checks whether or not an error has occurred in the communication data on the communication path. Since the client holds the secret information, the client determines that an error has not occurred, and obtains the communication data. On the other hand, a router, which does not hold the secret information, is unable to obtain the communication data. This prevents the communication data from being transferred to devices outside an authorized domain.