The edge-position detector 49 may search through a range in the digital image data along the conveying direction FD between a most upstream line, which is a first line to be read, and a 500th line inclusive in digital data pixel by pixel for edges of an original sheet GS in a method, for example, disclosed in U.S. Pat. No. 9,479,661 B2. When edges are detected in some of the pixels, the edge-position detector 49 may save edge data ED associated with the main-scanning positions MDP of the pixels in the RAM 42. More specifically, the edge-position detector 49 may search for an edge in each main-scanning position MDP, i.e., in each row of the pixels aligning along the conveying direction FD in the range between the most upstream line and the 500th line, and when an edge is detected for the first time in the row at the main-scanning position MDP, the edge-position detector 49 may save an edge position EP, indicating the position of the pixel containing the first-detected edge, as edge data ED in the RAM 42. Meanwhile, if no edge is detected at the main-scanning position MDP, i.e., in the row of the pixels aligning along the conveying direction FD between the most upstream line and the 500th line, the edge-position detector 49 may save a value “0,” which indicates “edge absence” or “no edge NE” as the edge data ED in association with the main-scanning position MDP in the RAM 42. The edge position EP may mean a position of a pixel, at which a color in the pixel indicated in the digital data shifted from white to black along the conveying direction FD. For example, as shown in FIG. 12A, when an edge is detected for the first time at the 160th line in the row at the main-scanning position “3,” the edge-position detector 49 may save a value “3” as the main-scanning position MDP and a value “160” as the edge data ED in the RAM 42. Meanwhile, when no edge is detected at the position of the foremost pixels, the edge-position detector 49 may save values “1” as the main-scanning position MDP and “0” indicating the absence of the edge or “no edge NE” as the edge data ED in the RAM 42.