An image reading device generates, as image data of a reading region, image data for output and image data for size detection, the latter having a lower resolution than the former, discriminates high density lines from the size-detection image data, sets as a target line one of the high density lines. And the image reading device calculates as a width for size detection a width from a reference line to the target line, sets as the size of a document a reference size that has a width closest to the width for size detection, performs image processing on the output image data based on the reference size set as the size of the document, and outputs the processed image data.