An image processing apparatus is connected to an image reading apparatus which reads a read area set on a scanner platen in accordance with a size of a document placed on the scanner platen so as to create image data in which an image of the document is shown. The image processing apparatus includes a document detection section and an instruction section. The document detection section creates, based on the image data supplied from the image reading apparatus, document area information indicative of an area where the document is placed on the scanner platen. In a case where the area indicated by the document area information is not completely encompassed by the read area, the instruction section sets a reread area which completely encompasses the area indicated by the document area information and causes the image reading apparatus to read the reread area. In a case where a document which is not completely encompassed by the read area is read, a user can carry out a rereading process more easily than a conventional art.