1. A pattern noise removal device which removes a background pattern noise having periodicity from a digital image, the pattern noise removal device comprising: a candidate pixel extracting module configured to extract candidate pixels that constitute the background pattern noise from pixels within the image based on density of each of the pixels; a dot pattern feature extracting module configured to extract a feature of a dot pattern formed by each of the candidate pixels and peripheral neighborhood pixels among the extracted candidate pixels; a dot pattern area extracting module configured to extract a dot pattern area including the periodic candidate pixels and neighborhood pixels having the feature of the dot pattern; a dot pattern categorizing module configured to, in a case of the feature of the dot pattern having tilt on the horizontal and vertical coordinates, analyze the feature of the dot pattern extracted by the dot pattern feature extracting module by each of the dot pattern areas extracted by the dot pattern area extracting module, and classify a plurality of dot pattern areas by each feature of the dot pattern through assembling the dot pattern area having a common dot pattern feature, with a restricting condition which is the dot pattern feature different by each dot pattern area; and a periodic pattern noise removal module configured to acquire, by having the dot pattern area classified by the dot pattern categorizing module as unit, information of the dot pattern area extracted by the dot pattern area extracting module and information of the feature of the dot pattern extracted by the dot pattern feature extracting module and remove a plurality of dot candidate pixels included in the dot pattern area as the dot pattern noise by having the feature of the dot pattern as the restricting condition.