Original document size detection using a line sensor for reading the original document

Abstract

In an image reading device, a reading unit scans first rectangle regions located within a maximum reading area equivalent to a maximum size of a document that can be scanned and acquires image data from the first rectangle regions. A transporting unit moves the reading unit to the first rectangle regions in a direction of movement opposite to a direction of movement in normal document reading. A document size judging unit detects whether a document exists in each first rectangle region based on the image data acquired by the reading unit and a predetermined threshold, so that a size of the document is judged. The reading unit is arranged to scan second rectangle regions located outside the maximum reading area, the second rectangles being different from the first rectangle regions, and acquire image data from the second rectangle regions.