An image processing method that differentiates image data using an image processing system that includes a processing unit and a storage unit includes: acquiring image data; sequentially picking up pixels one by one at a predetermined pitch from among the pixels that constitute the image data and setting the picked up pixels as reference pixels; setting a close region around each of the reference pixels; calculating an average value of densities of the picked up pixels for each of the close regions; setting a wide region larger than the close region around each of the reference pixels; calculating an average value of densities of the picked up pixels for each of the wide regions; and calculating a difference between the density of each of the reference pixels and a corresponding one of the average values of the densities of the pixels of the wide regions.