1. A mosaic image generating apparatus which generates a mosaic image by use of material images, comprising: a segmenting unit which segments a target image becoming a basis of the mosaic image into a plurality of blocks; a target image processing unit which calculates an average density value, as a target density value, of each of basic colors within each of the blocks into which the target image is segmented by said segmenting unit; a determining unit which determines a placement block in which the material image is to be placed from within the plurality of blocks without being restricted by an image of the block; a calculating unit which calculates the average density value of each of the basic colors within the material image; a color correcting unit which makes a color-correction of the material image so that the average density value of each of the basic colors within the material image becomes the target density value of each of the basic colors within the placement block while keeping a density value distribution ratio of each of the basic colors of the material image; and a target image correcting unit which compresses, in a state of uniforming each of the basic colors of the images of the respective blocks with a color set to the target density value of each of the basic colors that is calculated by said target image processing unit, the density value distribution ratio of each of the basic colors within the target image so as not to use a predetermined range extending from an allowable minimum density value and a predetermined range extending from an allowable maximum density value, and newly calculates, as the target density value, the average density value of each of the basic colors corresponding to the compressed density value distribution ratio, wherein said color correcting unit color-corrects the material image by using the target density value that is newly calculated by said target image correcting unit.