1. A method of storing document data in a filing system comprising the steps of: (a) storing data representing a processed image, which includes the content of an original document, and secondary data added to the document in a first memory region of said filing system; (b) comparing the processed image as stored in said first memory region with the image of the original document in order to detect portions which differ in respective images; (c) classifying the portions which differ as detected in the comparing step (b) according to predetermined classification standards; (d) specifying at least one local region of said processed image which has a predetermined relationship in position that is determined by a selected classification standard with respect to the portion which differs in the processed image; and (e) storing the data that represents said local region in a second memory region of said filing system as retrieval data together with a code that indicates the relationship of said retrieval data to said original document.