An ADSL line performance test portion implements an ADSL performance test before starting ADSL line service operation and grasps a subscriber line state. Test results are stored in a database and are referred to for the sake of maintenance of ADSL line service. In addition, the ADSL line performance test portion operates in a maintenance mode as required and also implements the ADSL line performance test in an ADSL line service operation state so as to constantly hold in the database a current optimum ADSL line configuration parameter for the subscriber line.