A method for image processing in which an area of a platen on which a document is placed is read and the platen image is processed includes extracting an object included in the platen image, detecting an area of the object and inclination of the area of the object with respect to the platen image, determining the minimum rectangular area which includes the area of the object, and is surrounded with four sides parallel to four sides of the platen image respectively, comparing an extent to which a pixel group constituting an edge of each area includes pixels constituting an edge of the platen image between the area of the object and the minimum rectangular area, and determining a minimum rectangular area as the area of the document if a pixel group includes a larger number of pixels based on the comparing or if the inclination is small.