1. An image matching device comprising: a feature point calculation section for calculating feature points on an input document image from inputted data of the input document image; a features calculation section for calculating features of the input document image in accordance with a relative position between the feature points calculated by the feature point calculation section; a similarity determination section for determining whether or not the input document image is similar to the reference document image, the similarity determination section performing the determination by comparing (i) the features of the input document image which are calculated by the features calculation section with (ii) features of a reference document image; and a document discrimination section for discriminating whether or not the input document image is an image of an N-up document if the similarity determination section determines that the input document image is similar to the reference document image, the document discrimination section, in accordance with coordinate positions of feature points on the input document image and feature points on the reference document image which coincides with the input document image in features, determining where on the input document image a position of the reference document image is located correspondingly, and the document discrimination section discriminating whether or not the input document image is the image of the N-up document with use of information on where on the input document image the position of the reference document image is located correspondingly.