PURPOSE: To keep off variations in machining as well as to make unattended operation performable, by letting a numerical control system select a machining condition if the desired surface roughness and machining accuracy are specified, while making it so as to control external units, in case of the numerical control system used for an electric discharge machine.

CONSTITUTION: Input items out of an input medium 12 are linear/circular arc commands, coordinate data, cutting speed, servo voltage, surface roughness and machining accuracy as information of material quality and thickness of a work, material quality and thickness of a wire electrode and a cutting pattern. A detector 14 detects the present value of each of parameters in order to perform process control as to liquid pressure or flow of a dielectric fluid. Data transmission between a numerical control system 13' and external units such as a machining power supply 8, a machine proper 1, etc., is entirely carried out via an input-output control part 15. A optimum control processing part 16 searches for an optimum condition among various machining conditions registered on the basis of input information 12, namely, the work, the service electrode wire specified surface roughness and accuracy, and performs their output processes, controlling these external units via the input-output control part 15.