In an input image generated as a binary image from a scanned document image, a projection image generating unit (a) tries at each primary-scanning-directional target pixel position to detect a secondary-scanning-directional run along a secondary scanning direction, projects the detected secondary-scanning-directional run at the primary-scanning-directional target pixel position and thereby generates a one-dimensional primary-scanning-directional-projection image, and (b) tries at each secondary-scanning-directional target pixel position to detect a primary-scanning-directional run along the primary scanning direction, projects the detected primary-scanning-directional run at the secondary-scanning-directional target pixel position and thereby generates a one-dimensional secondary-scanning-directional-projection image. A blank sheet determining unit determines that the document is a blank document at least on the basis of the number of runs in the primary-scanning-directional-projection image and the number of runs in the secondary-scanning-directional-projection image.