1. An image processing apparatus comprising: input means for inputting original image data showing an original image, wherein the original image data is constructed by a plurality of bit planes including a first bit plane and a second bit plane; compressing means for lossless-compressing the first bit plane and the second bit plane so that, in a case where the compressed first bit plane and the compressed second bit plane are composited, an amount of composite compressed data is smaller than a data amount of one uncompressed bit plane; data compositing means for compositing the compressed first bit plane and the compressed second bit plane to generate the composite compressed data; first substituting means for substituting the composite compressed data for the uncompressed first bit plane of the original image data; and second substituting means for substituting additional information for the uncompressed second bit plane of the original image data.