PURPOSE: To ensure the detection of the presence or absence or the degree of quality defect such as chatter, by projecting a light beam in a spot shape, which is larger than the periodic interval of cutting unevenness, and detecting the brightness of luminance of scattered light reflected from a cut and machined surface in a space.

CONSTITUTION: On a screw cutting part 10A of a steel pipe 10, a light beam, e.g., a laser light beam in a visible light region, in a spot shape, which is larger than the periodic interval of the cutting unevenness to be detected, is projected from a laser light oscillator 12, which is an oscillating source. The scattered light reflected from the screw cut surface is projected on, e.g., a white screen 14. When quality defect is present, a stripe shaped variable-density pattern appears on the screen 14 in the color corresponding to the color tone of the laser light. The image is processed, and the quality is automatically judged. The variable density of the stripe pattern is automatically detected using an electric signal by a linear array and the like, in which photosensors are used. Thus the quality can be automatically judged.