1. A learning device comprising: generating means configured to generate an image having different resolution from an input image; extracting means configured to extract a feature point serving as a processing object from an image generated by the generating means; calculating means configured to calculate the feature amount of the feature point by subjecting the feature point to filter processing employing a predetermined filter; and identifier generating means configured to generate an identifier for detecting a predetermined target object from the image by statistical learning employing the feature amount; wherein the filter includes a plurality of regions, and the calculating means take the difference value of difference within the regions as the feature amount.