1. A pattern separating extraction device comprising: a displaying section to display a document image that includes a plurality of print patterns that include a section that has a background color and a color of one of pre-print patterns of the plurality of print patterns or a section that has the background color and a color of one of post print patterns of the plurality of print patterns; an acceptance section to accept a section that has the background color and a color of one of the print patterns in the document image displayed by said displaying section as a color sample for each of the print patterns through an input device; an approximate straight line specification section to specify 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 section; a belonging degree calculation section to calculate 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 section; a belonging target determination section to determine 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 section to generate image data of an image that has 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 section; and an output section to output the image data about the respective print patterns generated by said generation section, wherein said approximate straight line specification section sets up a unit sphere that is centered on the coordinate point of the background color in said color sample in said color space, equally divides a spherical surface of said unit sphere into predetermined number of unit areas, and wherein said approximate straight line specification section specifies one unit area through which the maximum number of the straight lines connecting the coordinate point of the background color and a coordinate point of a color other than the background color pass, and specifies a straight line connecting said specified unit area and said coordinate point of the background color as the approximate straight line.