The leading edge and the trailing edge of an original document are detected with a sheet detection sensor provided in an automatic document feeding device, reading out of the original document is started from a position a predetermined distance before the position where the leading edge of the original document passes a reading position in an image reading apparatus, and reading out thereof is terminated at a position a predetermined distance after the position where the trailing edge of the original document passes the reading position. It is determined whether the apices of a rectangular area extracted from an image area that is read out are included in the respective areas of a predetermined height from the leading end and the trailing end of the image area to determine whether the size of the document image area is the same as the sheet size of the original document.