The maximum number of documents placeable on a document platen may be detected by comparing the document size and document platen size. Then, a number of copy sheets equal to the maximum number of documents is chucked on a transfer drum. When the documents containing color regions and document that does not contain color regions are mixed among the plurality of documents placed on a document platen, a single scanning operation is executed for all document regions for developing a black toner, and three scanning operations are executed for documents containing color regions for developing a cyan, magenta and yellow toner.