A technique extracts an object that is characteristic although the number of appearances is less demanded. A clustering apparatus includes: a similarity degree calculating section calculating a similarity degree of a combination of optional two of objects to store the calculated similarity degree in a similarity degree table, excluding a combination of one of the optional two and itself; a merging object selecting section selecting as merging objects, two objects related to the similarity degree which satisfies a predetermined reference; a new object generating section generating a new object from the merging objects; a merging object removing section removing from the similarity degree table, a similarity degree between each of the two objects selected as the merging objects and each of the objects; and a new object adding section calculating a similarity degree between the new object and each of the plurality of objects other than the new object.