PURPOSE: To decide the exactitude of the presence of defect by partitioning the surface to be inspected of a processed material into plural parts, by inspecting the surface for defect two times on corresponding each part in the former step to that in the latter step of the process and by logically deciding the presence of defect on the respective inspection data.

CONSTITUTION: In the surface defect detecting method of manufactured material both the dimension and shape of which are changed by processings such as rolling, forging and drawing, the inspected surface of rolled material treated by the bath which is the last but six is partitioned into the latticed face 1aWNa as shown by (a) in Figure. The latticed face is expanded to the one (b) 1bWNb by the next reciprocating bath, and is done to 1cWNc and to 1dWNd in turn. The respective patitions in the former and latter steps of the manufacturing process are made corresponding to each other and inspected for defect two times at least, the probability of the presence of defect is decided on the inspection data in the former and latter steps. In this way, the precision of detecting the defect can be improved.