1. An image processing method for processing an image obtained by reading a document plate on which at least one document is placed, comprising: a first detecting step of specifying, by a first detecting unit, a first rectangular region including all of objects included in the image to detect the coordinates of each vertex of the first rectangular region; an identifying step of identifying, by an identifying unit, on each of the objects whether the object corresponds to a document item serving as or included in the document placed on the document plate; an eliminating step of eliminating, by an eliminating unit, the object, identified as a non-object that does not correspond to a document item in the identifying step, from the image; a second detecting step of specifying, by a second detecting unit, a second rectangular region to detect the coordinates of each vertex of the second rectangular region, the second rectangular region including all of the objects in the image subjected to the eliminating step, each side of the second rectangular region being parallel to the corresponding side of the image, the second rectangular region having a minimum area; a comparing step of comparing, by a comparing unit, the coordinates of each vertex detected in the first detecting step with the coordinates of the vertices detected in the second detecting step; and a determining step of determining, by a determining unit, a method of cropping the document in accordance with the result of the comparing step.