PURPOSE: To efficiently automatically grind a screw slot of a spiral tap by obtaining the uppermost top position of cutting edge sequentially changing on the basis of dimensions of spiral tap while providing a control circuit for performing predetermined backing-off from said position in parallel to a NC device system.

CONSTITUTION: Dimensions input through MDI 27 and I/O control circuit 28, such as nominal diameter D, number of screw threads worked, pitch P, number N of slots, slot width θ, torsion angle α of slot, relief amount (d), number of index threads, etc., of a spiral tap 4 permit predetermined calculation to be performed to detect the uppermost top position and the next uppermost top position on respective threads of spiral tap 4. Then, a motor 8 for rotating the tap 4, a motor 13 for automatically feeding the tap 4 axially and a motor 18 for automatically feeding a grinding wheel 16 orthogonally to the axis of tap are respectively synchronously rotated with reference to the respective detected uppermost top positions. Thus, predetermined backing-off is performed on all cutting edges of tap 4.