PURPOSE: To detect a tapping depth with accuracy by measuring a vertical height variation with a linear displacement measurement system and comparing the data with the tapping depth stored in advance in a storage device with a measuring device.

CONSTITUTION: A tap 4 is positioned to a place where a cutting process is started, the momentary height value of which tap is measured by a displacement measuring system 7. This system scans slits 9 cut in a metallic plate 8 with its optical accumulation system, the pulses by which are counted up by a counter 17 for the computation of the tapping depth. The displacement measuring system 7 is connected to a microprocessor 15. An operation of an actuating key simultaneously resets the momentary height value of the tap 4 to zero, and after a slight timelag a cutting process is started. The actual penetrating depth of the tap 4 is next measured and compared with a value indicative of a desired tapping depth stored in a storage device 16, in which state when reaching the depth, the tap 4 is rotated in the reverse direction so as to be unscrewed away from the threaded part.