A multifunction peripheral (MFP), which successively conveys a plurality of documents stacked on a document tray and reads the document, determines whether there is a difference among sizes of the plurality of documents in response to at least one of the documents being read by a scanner, in a case where a preview operation of a read image is performed, notifies a user of a possibility that documents having different sizes are mixed and read based on a result of the determination, enables a function of performing reading corresponding to respective document size of mixed size documents in which documents having different sizes are mixed, and provides navigation for prompting re-reading of the document.