PURPOSE: To reduce dispersion in facial accuracy of a processed surface and thicknesses, by a method wherein a displacement quantity of a rotary axial direction of a cutting tool is detected by a plurality of displacement detectors, an actual displacement quantity of a cut off part is calculated through operation and controlled so that the displacement quantity becomes zero.

CONSTITUTION: The first displacement detector 41 is installed most close by an ingot 34 and a displacement quantity A of a blade 51 is detected. The second displacement detector 42 installed to a position separated from the ingot 34 and a displacement quantity B of the blade 51 is detected. The maximum displacement quantity of the central part of a cut point and an inclination angle of the blade 51 are calculated in consideration of both the detected quantities A, B, a space up to the center of both the displacement detectors 41, 42 from the center of a cutting width of a cutting edge 52 of the blade 51 at the time of cutting of the ingot 34 and rigidity to bending of the blade 51. Control is performed so that both of them become zero. With this construction, control is performed so that displacement of a cutting tool is not generated substantially and a work is free from dispersion in thicknesses and facial accuracy of a processed surface is improved.