1. A computer readable medium containing a pattern separating extraction program that controls a computer to execute functions comprising: a displaying function for displaying a document image that includes a plurality of print patterns that consist of one post print pattern and one or more post print pattern and/or one or more pre-print pattern on a display device; an acceptance function for accepting a section that consists of a background color and a color of one of the print patterns in the document image displayed by said displaying function as a color sample for each of the print patterns through an input device; an approximate straight line specification function for specifying an approximate straight line of a distribution in a color space about a color other than the background color for each color sample that is specified by each designation accepted by said acceptance function; a belonging degree calculation function for calculating a belonging degree of each of all the pixels in the document image to each of the print patterns based on a distance between a color of the pixel and each of the approximate straight lines that are specified by said approximate straight line specification function; a belonging target determination function for determining a print pattern as a belonging target for each of all the pixels in the document image when the belonging degree of the pixel to the print pattern exceeds a predetermined threshold; a generation function for generating image data of an image that consists of only one print pattern based on the pixels that belong to the print pattern for each of the print patterns in which the pixels belong thereto are determined by said belonging target determination function; and an output function for outputting the image data about the respective print patterns generated by said generation function.