1. A clustering apparatus comprising: a processor; a computer memory; a similarity degree calculating section configured to calculate a similarity degree of a combination of optional two of a plurality of objects to store the calculated similarity degree in a similarity degree table in the computer memory, wherein the combination excluding combining of one of the optional two objects and itself, and the similarity degree being calculated based on an approximate calculation method of Kolmogorov complexity; a merging object selecting section configured to select as merging objects two related objects having a particular similarity degree which satisfies a predetermined reference; a new object generating section configured to generate a new object from the merging objects by concatenating the two related objects selected as the merging objects, wherein the new object fully incorporates each of the two related objects; a merging object removing section configured to remove from the similarity degree table, the particular similarity degree between each of the two related objects selected as the merging objects and each of the plurality of objects; and a new object adding section configured to calculate a new similarity degree between the new object and each of the remaining of the plurality of objects other than the new object, wherein the selecting, the generating, the removing are repeated with respect to the new object based at least on the calculated new similarity degree satisfies the predetermined preference, the processor causes at least one of the similarity degree calculating section, the merging object selecting section, the new object generating section, the merging object removing section, and the new object adding section to perform the respective calculating, selecting, generating, removing, and calculating, and each of the plurality of objects is document data.