This communication system ensures the security of a communication message with a low computational load. Communication units are capable of exchanging a communication message containing a check bit. The check bit is used to determine the reliability of the communication message. Communication unit, which transmits the communication message, is equipped with a storage position determination part. The storage position determination part determines one of multiple positions within the communication message where the check bit can be stored as a storage position. The communication unit generates a communication message with the check bit stored at the storage position and transmits the communication message. Communication unit, which receives the communication message, is equipped with a storage position determination part. The storage position determination part determines the storage position of the check bit in the communication message in accordance with the manner the storage position was determined.