PURPOSE: To invariably maintain a high working accuracy through automatic correction of the numerical control command itseld based on the resuts in the projection of errors due to geometrical factors and deformation.

CONSTITUTION: A measuring action command is provided to a minicomputer 9 from a measurement part program 9, under the control of CNC (a computer- aided numerical control equipment) 14 as directed by the computer 9, an intersectional dimension sensor 5 or a rotational dimension sensor 6 combines to measure the dimensions of a masterpiece 7. The measured data and the known dimensions of the master piece are compared to compute errors. The part program 11 is so corrected that the errors is blow a given value whereby a corrected part program 12 is prepared. In this manner the command of CNC can be corrected. This enables the working of the work at a constant accuracy.