1. An image processing apparatus which compares features extracted from inputted document image data with features of a reference image stored in advance to perform a judgment process on whether the inputted document image data is similar to the reference image stored in advance, wherein when points indicating local characteristics of the image of the inputted document image data, which are determined from plurality of pixel values in the inputted document image data, are extracted and regarded as feature points, the features are values calculated on the basis of the positional relationship among plurality of feature points selected around a certain target feature point; and the image processing apparatus comprises a neighbor point selection section for, when selecting the plurality of feature points, selecting a predetermined number of feature points after excluding a prespecified number of feature points in turn from the feature point nearest to the target feature point.