1. A feature descriptor encoding apparatus comprising: a feature point detection unit configured to detect feature points from an image and outputs coordinate values; a local feature descriptor extracting unit configured to extract local feature descriptors from local regions centered on the coordinate values of the feature points; a coordinate value scanning unit configured to convert the coordinate values into index values according to a specific scanning method; a sorting unit configured to sort the index values; a difference calculation unit configured to calculate a differential value between two adjacent index values of the sorted index values and output a sequence of differential values; a differential encoding unit configured to encode the sequence of the differential values in a sequential permutation; and a local feature descriptor encoding unit configured to encode the local feature descriptors of the corresponding feature points in the same permutation as that of the sorted index values.