1. An image processing apparatus connected to a server, the image processing apparatus comprising: a document image reading section configured to read, among documents constituting a book, an image of a document of at least one page of the book; and a control circuit that includes a CPU and, based on operation of the CPU in accordance with a program, operates as: a document image checking section configured to (1) OCR the read image of the document read by the document image reading section to detect a plurality of characters existing in the read image and acquire, among the plurality of characters, a predetermined number of characters beginning with a character located at a predetermined location, (2) acquire, among a plurality of characters contained in each of registered images of respective documents stored in the server, the same predetermined number of characters beginning with a character located at the predetermined location, and (3) determine whether or not the characters acquired from the read image match the characters acquired from the one registered image, thus determining whether or not the registered image matching the read image exists in all of the registered images of the documents stored in the server; a document image acquisition section configured to, when the document image checking section determines that the registered image matching the read image exists, acquire the matching registered image; and a document image printing section configured to form an image on a paper sheet using the registered image acquired by the document image acquisition section.