PURPOSE: To determine the quality of knurled portions regardless of the surface states of screw threads by measuring the bright and dark portions of a screw along the screw axis and a two-dimentional direction perpendicular to the screw axis on a photographic image of the surface of the screw.

CONSTITUTION: A screw 16a is illuminated by a light source 22 and a camera device 24 photographs its surface and picks up its image. In the image, a helically threaded portion A appears bright and the recessed portions and the projecting portions of a knurled portion B make dark portions 30 and bright portions 32, respectively. A starting address Y1 is calculated as the measurement starting point of X, Y coordinates within the frame 36 of the knurled portion B. In the address Y1, the line of the bright portions and the line of the dark portions are traced from the right to the left of the frame 36 so as to calculate the Y-direction address of the width of each line. The line widths of the bright portions and the dark portions are evalulated in connection with the difference between the maximum value and the minimum value of X- coordinate addresses X1-Xn measured at the threaded portion A, and when the line address of each line width is greater than the difference the knurled portion B is judged as being nondefective, whereas when any of the line addresses is smaller than the difference the knurled portion B is judged as being defective.