An image reading apparatus to read image of rectangle shape document set on a setting board, is supplied capable of assigning correct edge image to an output image. In the image reading apparatus, a image data storing section stores image data reading area lager than the rectangle shape document; an edge feature extracting section extracts edge feature according to the image data; a rectangle feature extracting section extracts edge feature of side regions respectively corresponding to each side of the rectangle shape document according to the edge feature; a region selecting section selects two regions from the side regions; a coordinates calculating section calculates coordinates specifying position of straight lines representing four sides through using inclination information of the selected respective two regions; and a compounded-image outputting section replaces the feature region with frame image according to the coordinates, compounds the frame image with image data, and outputs the compounded image.