PURPOSE: To enhance the machining accuracy by measuring the actual shape of a work quickly and precisely by the on-machine system, and correcting the machining error by the in-process control on the basis of the data acquired from measurement.

CONSTITUTION: A laser interferometer 9 not likely to be influenced by vibration and the straightness in movement of the guide surface is installed on the X-axis table 5 of a super-precision working lathe. The shape of the machining surface of a work 12 under machining is measured by this laser interferometer 9, and the acquired data is fed to an NC device 19 through a processor 16. The NC device 19 prepares the machining error correction data on the basis of this measuring data, to serve for controlling servo motors 7 and 10 which drive the spindle stock 6 and tool rest 8, respectively.