1. A data processing device, comprising: difference value computing means that computes a difference value between a pixel value of a target pixel that is each pixel contained in an image and a pixel value of a pixel that is present at a predetermined neighboring relative position of the target pixel; representative value computing means that extracts a pixel group containing pixels that are similarly influenced by shading due to light from the image with respect to each pixel of the image and computes a representative value of difference values of the pixel group according to a statistical technique; feature value computing means that computes the feature value with respect to each pixel contained in the image based on comparison between the difference value with respect to each pixel and the representative value of difference values of the pixel group; and similarity deter mining means that determines a similarity between the image and a predetermined image based on the feature value with respect to each pixel extracted by the feature value computing means.