1. An image processing apparatus, including a computer-readable storage medium encoded with a computer program which is executable on a computer for retrieving retrieval-target image data that is similar to an entered document image, which operates as a scanner and a printer, comprising: a scanning unit for scanning a document and generating the entered document image; a segmentation unit for segmenting the entered document image and the retrieval-target image data into a plurality of areas on a per-attribute basis; an area specification unit for specifying an area to be emphasized of the plurality of areas segmented by said segmentation unit based on a user's instruction; a detection unit for detecting pointer information in a predetermined area of the plurality of areas of the entered document image, which indicates storage location in a storage device storing original image data of the entered document image; a first image data retrieving unit for retrieving the original image data of the entered document image based on the pointer information if the pointer information is detected by said detection unit; and a second image data retrieving unit for retrieving image data that is similar to the entered document image if the pointer information is not detected by the detection unit, including 1) a layout-similarity calculation unit for calculating a layout degree of similarity, which is a degree of similarity between a layout of areas obtained by segmentation in the entered document image and a layout of areas obtained by segmentation in the retrieval-target image data, and 2) a similarity calculation unit for calculating a degree of similarity, with regard to the retrieval-target image data for which the calculated layout degree of similarity is greater than a predetermined threshold value, for every area obtained by segmentation, using a comparison unit suited to the attribute; and an overall-similarity calculation unit for calculating an overall degree of similarity, with regard to the retrieval-target image data for which the calculated layout degree of similarity is greater than a predetermined threshold value, based on the degree of similarity calculated for every area obtained by segmentation and a weighting coefficient corresponding to the degree of similarity calculated for every area obtained by segmentation, wherein the weighting coefficient is calculated depending on a ratio between a sum of sizes of all segmented areas in the entered document image and each size of each segmented area in the entered document image, and is increased if the area corresponding to the weighting coefficient is the area specified by the area specification unit and decreased if the area corresponding to the weighting coefficient is not the area specified by the area specification unit, and wherein the entered document image is converted to vector data by a vector data conversion unit and registered as retrieval-target image data if the overall degree of similarity calculated by the overall-similarity calculation unit is lower than a predetermined threshold, and wherein said vector data conversion unit converts the document image to vector data in a case where an original data file corresponding to the entered document image could not be found based upon a result of calculation by said overall-similarity calculation unit; and a storage unit for storing the entered document image that has been converted to the vector data.