1. An image processing apparatus for performing an image quality improving processing of an image, comprising: a domain block extracting section for extracting a domain block image from an original image in the unit of a first block unit; a range block extracting section for extracting a range block image from the original image in the unit of a second block unit which is larger than the first block unit with respect to the domain block image; a reduced range block forming section for reducing the extracted range block image to the size of the first block unit; an improved domain block forming section for performing a pixel value conversion with respect to the reduced range block image formed by the reduced range block forming section, and for outputting the pixel-value-converted reduced range block image as an improved domain block image; a similarity degree judging section for judging a similarity degree between the domain block image and the reduced range block image by the reduced range block forming section, the improved domain block forming section performs the pixel value conversion based upon the similarity degree obtained by the similarity degree judging section; and a domain block classifying section for classifying a sort of the domain block image extracted by the domain block extracting section, wherein the domain block image other than the domain block image which has been classified to a previously determined sort is directly outputted as the improved domain block image, the domain block classifying section classifies the domain block image to a flat portion, a step edge portion, a noise portion, and a texture portion based upon both standard deviation and a concave/convex degree of the domain block image, and the each of the above sections is implemented in hardware or hardware/software combination.