1. An image processing apparatus, comprising: feature data extraction means for extracting feature data indicative of density characteristics of document based on image signals obtained by scanning the document; image recognition means composed of a neural circuit network which is learned beforehand so as to recognize image characteristics of the document based on the feature data; and density correction means for correcting the density of the image signals in accordance with the image characteristics based on image recognition information from said image recognition means, wherein said feature data extraction means outputs information indicative of a density distribution of the document based on the image signals as the feature data, wherein said information indicative of the density distribution of the document is density histogram information, and further including means for reducing the density histogram information while maintaining the information indicative of the density distribution, wherein the density histogram information is reduced based on a plurality of threshold values being set in stages.