PROBLEM TO BE SOLVED: To provide a scanner capable of quickly specifying the size of a document to be scanned. SOLUTION: In a scan image, a reference line and A, B detection lines are provided, where the reference line corresponds to the center line in a width direction of a conveying passage of a document, and the A, B detection lines are positioned at both the sides of the reference line. Then, when scan image data for a front edge part of the document are generated, edges of the document are detected on the A, B detection lines, and the respective edges on the A, B detection lines are set to be points a, b, respectively. Further, the edge detected on a line passing through a point positioned at a lower side in the points a, b and extending in an X-axis direction is set to be a point (point c) on a side edge of the document, and the coordinates of the intersection (point d) between a straight line C passing through the point c and orthogonally crossing a straight line ab, and the straight line ab are specified as coordinates of the vertex of the document. Then, the size of the document is estimated based on distance in the X-axis direction between the point d and the reference line. COPYRIGHT: (C)2011,JPO&INPIT