To accurately measure the displacement in the vicinity of a blade part and to prevent the interference of the blade part with an initial cut piece by setting the detection position of a displacement detector to the almost central part of a cutting passing width at the time of usual cutting and moving same to a position shifted from the cutting passing width at the time of initial cutting.

The detection position of a displacement detector 10 is set so as to become the central position P of ingot passing width in the vicinity of the blade part 25a of a blade at the time of usual cutting. At this time, the distance between the detection surface of the displacement detector 10 and the blade is within the detectable distance of the displacement detector 10 and blade displacement can be accurately detected. At a time of initial cutting, the rotary cylinder of a detector driving part 40 is driven to move the displacement detector 10 in a Y-direction by L and, even if a cut piece having a thickness larger than that of a wafer is generated under the blade, it does not interfere with the displacement detector 10. Since the distance between the detection surface and the blade is not changed even if the displacement detector 10 is moved, blade displacement can be detected even at the time of initial cutting.