1. A method of compiling an image database storing reference images to be compared with a query image and being used for specific object recognition, the method comprising: an extracting step of extracting a plurality of reference feature vectors representing local features of different locations from a reference image to be stored into the image database, each reference feature vector having a vector length and a vector direction; a clustering step of forming clusters, each cluster being composed of different feature vectors; a selecting step of selecting a feature vector from each cluster as a representative vector of each cluster; and a storing step of storing the representative vector into the image data base for specific object recognition in relation with the reference image, wherein: the clustering step forms each cluster so that the reference feature vectors representing local features located closely within the reference image belong to the same cluster; the selecting step gives a priority to any of the reference feature vectors each having a longest vector length when selecting the representative vector; the comparison is made by generating a plurality of query feature vectors from the query image in the same manner as that of extracting the reference feature vector and adopting a near neighbor search between each of the query feature vectors and the representative vector of each cluster; and each of the above steps is executed by a computer.