1. An image processing apparatus comprising: at least a processor and at least a memory coupled to the at least the processor and having stored thereon instructions which when executed by the at least the processor cooperate to act as: an extraction unit configured to extract a feature information from an object in input image data; a registration unit configured to classify a plurality of feature information into a plurality of groups based upon a similarity, and register 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 by the extraction unit; and a deletion unit configured to delete, 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 deletion unit deletes, from the dictionary, the feature information of the group which does not reach the predetermined use level.