A communications line monitoring system of the present invention comprises a plurality of relay apparatus and a communications line monitoring apparatus for monitoring a line quality of a communications line of relayed data, and each of the relay apparatus comprises error detecting unit for detecting errors in received data, previous error detection determination unit for determining whether or not error detection has already been performed in other apparatus based on previous error detection information of the received data, and initial error detection process unit for, only when an error is detected and the error was undetected in other apparatus, autonomously notifying error detection information to the communications line monitoring apparatus, and for adding previous error detection information to data for transmission.