An image of an inspection target having a concave and convex pattern is picked up in an off-focus state by an image pickup element 4. Further, the image of the inspection target picked up in the off-focus state is taken in by an image processing computer 6. Based on the image, the image processing computer 6 prepares a light intensity profile having a peak corresponding to a boundary portion between a concave portion and a convex portion of the concave and convex pattern of the inspection target. For example, a change of the width of the concave or convex portion of the concave and convex portion can be detected very accurately, by measuring the width of the concave or convex portion of the concave and convex portion on the basis of the intensity profile.