PURPOSE: To obtain a segment detection method which enables the detection of even a long segment accurately as being relatively obscure but recognizable as such with human eyes.

CONSTITUTION: This method is to detect a segment existing in an image based on a density data indicating the degree of the density of pixels in the image to be inspected. The directivity and intensity of the pixels are determined separately for a small area made up of a plurality of the pixels to judge continuity by using symbol marks 111, 112... indicating directivity respectively or others. In this manner, the directivity and intensity are calculated for each of small area groups A and B continuing in directivity between adjacent small areas in terms of the group. The results of calculation as exceeding a specified level are extracted as segment.