A numerical control system diagnoses and maintains a numerical control machine tool by measuring backlash and downloading a backlash correction value to the numerical control machine tool. The system includes a host computer device that downloads a monitor management program to a numerical controller of the numerical control machine tool, collects numerical control information input/output between the numerical controller and a servomotor, analyzes the numerical control information, and diagnoses the numerical control machine tool.

A host computer device downloads a monitor management program to a numerical controller of an NC machine tool in step S10, collects numerical control information input/output between the numerical controller and a servomotor in step S30, analyzes numerical control information monitored in step S40, and diagnoses the NC machine tool in steps S50 to S110.