1. A contour-information extracting method by use of image processing for extracting desired contour information out of a plurality of pieces of contour information included in an image, the method including the steps of: prompting setting of a lower limit value for eliminating contour information with a length not larger than a predetermined length out of the plurality of pieces of contour information included in the image; and selecting contour information larger than the set lower limit value sequentially from the contour information with a shorter length.