A knowledge base unit (IB) stores expert knowledge for finding the causes of troubles, and knowledge for retrieving DI/DO data (IOD) to be exchanged between an NC apparatus (11) and a machine tool (13) and NC internal data (ITD). In case trouble develops and the content of trouble is input from an alarm detect unit (11g), a deduction mechanism (ADPR) automatically reads the DI/DO data and NC internal data (IOD, ITB) using the expert knowledge corresponding to the content of trouble, grasps the phenomenon that is really taking place based on these data, recognizes the cause of trouble from the above phenomenon, and displays the cause of trouble and a method to cope with it.