1. An image feature extractor for outputting a feature information data set of a pattern image including data of a feature vector of the pattern image and confidence estimation information of the feature vector attributed to each component of the feature vector, said image feature extractor comprising: a feature vector extraction means for extracting the feature vector from the pattern image; at least one quality index extraction means for extracting quality index information of the pattern image; an error distribution information storing means, each corresponding to each of said at least one quality index extraction means, wherein error distribution information is prepared, said error distribution information including correlation between variance or standard deviation of error distribution of a plurality of feature vectors extracted by said feature vector extraction means and the quality index information extracted from respective one of said at least one quality index extraction means corresponding to said plurality of feature vectors; a confidence attribution means, each corresponding to each of said error distribution information storing means and obtaining confidence information referring to said error distribution information prepared in respective one of said error distribution information storing means by way of said quality index information extracted from the pattern image by respective one of said at least one quality index extraction means, said confidence information being output as the confidence estimation information when said at least one is one; and a confidence estimation means provided when said at least one is more than one for obtaining the confidence estimation information by integrating said confidence information obtained by each of said confidence attribution means.