1. 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, the method comprising: 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 a 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 the 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.