1. An image processing apparatus that includes a similarity determination process section for comparing features of an image that are extracted from image data of a document to be matched and features of an image of a reference document that has been stored, so as to determine whether the image of the document to be matched is similar to the image of the reference document, the image processing apparatus comprising a conversion coefficient calculation section for determining whether the document to be matched that is determined by the similarity determination process section as being similar to the reference document has been zoomed with respect to the similar reference document, and when the document to be matched has been zoomed, the conversion coefficient calculation section calculating a conversion coefficient with which the size of the document to be matched is changed to the size of the similar reference document.