1. An image recognition method in which a mask pattern indicating characteristics of an image pattern to be recognized is used to recognize an isolated image isolated from target image data having a plurality of images to be recognized, the target image data having digitized gradations of two or more values and a same configuration and size, said method comprising: generating, by a mask pattern generation unit, the mask pattern to have a first pattern area which is smaller than the image to be recognized and is not positioned in a center portion, and a second pattern area which is positioned outside the first pattern area, is larger than the image to be recognized, and is smaller than a predetermined distance to adjacent other images to be recognized; recognizing, by a mask pattern generation unit, an area surrounded by the first pattern area as a fixed area, and an area sandwiched between the first pattern area and the second pattern area as a binarized area; obtaining, by a largest value detection circuit, a first representative value, which is a largest value of a luminance of a target image data, from the first pattern area; obtaining, by a smallest value detection circuit, a second representative value, which is a smallest value of a luminance of the target image data, from the second patter area; judging, by a comparison circuit, that an image to be recognized is present in the target image when a difference between the second representative value and the first representative value is equal to or larger than a predetermined threshold value; and comparing, by a binarization circuit, the target image data in the binarized area with a predetermined threshold value to binarize them when an image to be recognized is judged as being present in the target image.