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; and 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.