1. A control method for an image scanning device that creates image data by scanning an image from a recording medium on which the image is recorded, comprising the steps of: controlling said image scanning device to acquire first image data by scanning the image at a first scanning resolution; judging whether an image type indicated by the first image data is a type including an artificial image or a type including a natural image; rechecking whether the image type judged by the judging as the type including the artificial image is the type including the artificial image or the type including the natural image based on a criteria different from a criteria of the judging, and rechecking whether the image type judged by the judging as the type including the natural image is the type including the artificial image or the type including the natural image based on a criteria different from the criteria of the judging; classifying the image type judged by the rechecking as the type including the artificial image into a detailed sub-class depending on whether ground color of the image includes chromatic color, and classifying the image type judged by the rechecking as the type including the natural image into a detailed sub-class depending on a level of brightness of the image; determining a cutoff value specifying a gradation value of a shadow side being converted into a minimum value at a contrast enhancement process and a cutoff value specifying a gradation value of a highlight side being converted into a maximum value at the contrast enhancement process depending on the sub-class of the image type of the first image data; controlling said image scanning device to acquire second image data by scanning the image at a second scanning resolution; and performing the contrast enhancement process on the second image data based on the determined cutoff values.