1. An image processing device that processes image data representing a multi-tone image by multivalue coding and outputs results of the multivalue coding, said image processing device comprising: a pixel group tone value specification module that collects every preset number of multiple pixels to one pixel group to divide a large number of pixels constituting the image to plural pixel groups, and specifies a pixel group tone value as a representative tone value of each pixel group; a correlation map storage module that stores in advance plural correlation maps, each correlation map setting a restricted number of multivalue codes obtained as results of the multivalue coding in correlation to pixel group tone values, where the restricted number of multivalue codes is less than a total number of possible states, one of which is selectively specified for each pixel group based on a preset number of threshold values provided corresponding to the preset number of multiple pixels included in the pixel group; a multivalue coding module that selects one correlation map for each pixel group among the stored plural correlation maps and refers to the selected correlation map to convert the specified pixel group tone value of the pixel group to a multivalue code; and an output module that outputs multivalue codes of all the plural pixel groups.