1. An image processing apparatus, comprising: a control unit and an output device, wherein the control unit includes a mark detecting unit that, when a document on which reading processing is performed is read by an image reading apparatus along with an identification mark, detects the identification mark from an image of the document read; a position detecting unit that, when the identification mark is detected by the mark detecting unit, determines which portion of the document is read from the image of the document; and a notification output unit that outputs notification information indicating a read position to be read next time to the output device on the basis of which portion of the document is read as determined by the position detecting unit, wherein the identification mark is inserted at a top edge of the document along a document fold line, outside the document area and the identification marks is identified based on color and/or pattern.