PURPOSE: To accurately judge the cleanliness of the surface of a steel material on the basis of a definite judging standard before painting or surface treatment.

CONSTITUTION: The perfectly derusted surface of a steel material is subjected to binary-coded image processing to obtain image data which is, in turn, preliminarily stored in an image recognizing device 3 as a judging standard. The image data of the surface of the steel material 1 is detected by a television camera 2 and subjected to binary-coded image processing by the image recognizing device 3 using the preliminarily stored binary-coded image processing data for the judging standard to discriminate between a clean part and an unclean part and the result is formed into an image by a monitor television 4. At the same time, the area ratio of the clean part and the unclean part is operated by the image recognizing device 3 to be displayed on the monitor television 4.