PURPOSE: To detect whether a work has a periodic projection and recess surface or not and the value of the period by detecting the cutting feed speed in cutting work, turning on and off a fiber light source at a period 1/2W1/5 time as long as the feed period calculated on the basis of work conditions and mechanical oscillation characteristics and irradiating the surface of the work, and measuring the quantity of reflected light.

CONSTITUTION: To detect the periodic projection and recess state of the work, the detected period of projections and recesses should be equal to the flashing period of the light source when a table 1 on which the work 2 is placed moves at a constant speed. For the purpose, a value α calculated from transfer number 60.f/(n.z)=N±α (N: integer, α: end number) of oscillations of the work by one edge which is calculated on the basis of the natural oscillation frequency fHz and main spindle rotational frequency n(rpm) of a machine main spindle system and the number of cutting edges is utilized and the flashing period of the light source is set according to the value and the feed speed of the table; and the set value is shifted successively by a 1/10 period up to ±55/10 period to check whether there is periodicity or not. When the flashing period of the light source coincides with the projection-recess period of the work, the period of projections and recesses is calculated from the flashing period value.