When a document detector provided on a document tray is ON (S11: YES), a controller judges whether or not a document is of a standard size, on the basis of a document size table stored in a storing unit (S12). When the document is of a standard size (S12: YES), after a predetermined time has elapsed (S13: YES), the controller elevates/lowers the document tray on the basis of a detection output of a guide-in roller position detector and causes the document tray to wait in a state capable of supplying and conveying the topmost layer of documents immediately. An image forming apparatus thus constructed can make a judgment as to whether or not to execute supply and conveyance of documents, according to the mount state of the documents.