12. The method according to claim 11 , further comprising determining the similarity by referring to the dictionary.
11. An image processing method comprising: extracting a feature information from an object in input image data; classifying a plurality of feature information into a plurality of groups based upon a similarity, and registering the plurality of feature information into a dictionary, wherein the dictionary is used to determine a similarity between the feature information registered in the dictionary and the feature information extracted in the extracting; and deleting, among the plurality of groups, a feature information of a group, which does not reach a predetermined use level, from the dictionary, wherein information related to a time period elapsed from when the feature information had been registered in the dictionary at first is used as the use level, and wherein, in a case where the number of feature information registered in the dictionary exceeds a predetermined number, the feature information of a group which does not reach the predetermined use level is deleted from the dictionary.