Abstract: A system for diagnosing failures of ADSL comprises an access module for locating and monitoring the subscriber lines providing the ADSL services, a plurality of ADSL test devices each having various extension test boards corresponding to the test kinds detected for the failures of the subscriber lines accessed by the access module, and a control device for controlling the ADSL test devices. Preferably, extension boards may be additionally provided to adapt for increase of the number of the subscribers.