1. An image reading apparatus for reading to read an image of a document which has a rectangular shape, comprising: a storing section for storing image data obtained by reading an area larger than said document; a rectangular outline information extracting section for extracting rectangular outline information including four sides that correspond to four sides of the rectangular-shaped document according to the image data; a unit area calculating section for calculating respective unit areas per standardized length for each of said four sides in the rectangular outline information; a region selecting section for selecting outline information corresponding to two sides intersecting each other from said four sides of the rectangular outline information according to the calculated unit areas for each of the four sides; and a compounded-image outputting section for forming a rectangular outline image that corresponds to the four sides of the rectangular-shaped document based on the outline information of the selected two sides of the rectangular outline information, modifying said image data by removing a portion of the image data that corresponds to a rectangular outline region of the image data, and compounding the rectangular outline image with said modified image data, and outputting the compounded image.