PURPOSE: To provide the excellent cut surface by preventing the formation of a large level difference by measuring the tool length at a prescribed time interval or at each prescribed cutting distance and renewing the actual tool correction quantity in a certain interval by a prescribed quantity, so that the tool correction quantity corre sponding to the result of the measurement is obtained.

CONSTITUTION: A correction judgement system setting device 1 sets the correction judgement system for the cutting time or cutting distance. Further, in a cutting time counter 13 and a cutting length counter 14, the cutting time and the cutting length are counted. Further, in a measurement execution judging device 15, a tool length measurement execution signal is outputted into a tool length measurement execution device 16 when each count value exceeds each memory value of a time measurement interval memory device 5 and a cutting length memory device 6. In a correction execution quantity calculating device 18, the correction execution quantity is calculated on the basis of each memory value of a correction quantity variation memory device 12, correction quantity varying time memory device 9, or a correction quantity varying cutting length interval memory device 10, and tool correction is executed by a tool correction executing device 19. Accordingly, cutting is carried out with high precision, without generating the level difference on the cut surface.