PURPOSE: To improve productivity at the time of conversion by comparing plural data groups such as documents, detecting common data and forming a model of the documents.

CONSTITUTION: Plural data groups stored in a 1st storage means 2 are image data defined so that one page is one data group and a detecting means 3 detect common image data from the image data corresponding to two pages in these image data and stores the detected image data in a 2nd storage means 4. When image dates in two pages or more exist, the image data in each two pages are successively stored to detect the common image data and the image data common to all pages are stored in the 2nd storage means 4. Consequently, the image data common to all the pages are pages are stored in the 2nd storage means 4 and form overlay can be formed only in the ruled line information of documents or the like. Thus, productivity at the time of system conversion can be improved.