An image-reading processing apparatus is connected to an image reading apparatus being capable of reading both surface images of a document, including a storage unit and a control unit, wherein the control unit includes a double-face reading control unit that controls the image reading apparatus to scan both the surfaces of the document, a coordinate acquiring unit that acquires a coordinate group of a position serving as an edge candidate of the document based on a grayscale value in image data of each of the surfaces read by the double-face reading control unit, a straight line detecting unit that detects a straight line group corresponding to an edge portion of the document based on the coordinate group acquired by the coordinate acquiring unit, a straight line inverting unit that inverts the straight line group detected by the straight line detecting unit, and a straight line selecting unit that determines the straight line group suitable for the edge portion from the document in the straight line group detected by the straight line detecting unit on one surface and the straight line group inverted by the straight line inverting unit on the other surface to select the straight line group.