A scanning apparatus is controlled to extract edges of an original document from an image scanned by a scanning device, execute an end obtaining process which causes the scanning apparatus to obtain a leading end of the original document and a trailing end of the original document based on the edges extracted in the extraction process, an inclination obtaining process to obtain a first inclination of the leading end and a second inclination of the trailing end, and a determination process to determine that a rotational skew has occurred when a difference between the first inclination and the second inclination is greater than a rotational skew threshold, the rotational skew being a change of inclination of the original document during conveyance of the original document.