1. A method for detecting a defective image comprising: obtaining an edge image by performing an edge tracking process on an original image; obtaining a processed image by performing an erosion process after an expansion process on the edge image; detecting an isolation point having a size equal to or smaller than a predetermined size from said processed image; and storing a position of the isolation point in the processed image, and detecting an image in the original image that corresponds to the position as the defective image, wherein a rectangular region that surrounds the isolation point is determined, when a ratio of an area occupied by the isolation point to an area of a square whose side length is equal to a longer side length of the rectangular region is larger than a predetermined value, the position of the isolation point having a ratio smaller than a first threshold value is stored, when the ratio is smaller than the predetermined value, the position of the isolation point having a ratio smaller than a second threshold value is stored, and the first threshold value is smaller than the second threshold value.