1. An information processor carrying out statistical natural language processing for a document, the information processor comprising: characteristic amount extraction means for detecting context information including a proper noun pair from the document and extracting a characteristic amount of the detected context information; characteristic amount analysis means for, by analyzing the characteristic amount of the extracted context information using a probability model in which a document topic meaning an entire topic of the document and a context topic meaning a local topic of the document are considered, estimating a potential variable and a context topic ratio in the probability model; and clustering means for clustering the proper noun pair included in the context information based on the context topic ratio estimated regarding the characteristic amount of the respective context information, wherein the characteristic amount extraction means includes: detection means for detecting a proper noun in the document; anaphora analysis means for carrying out an anaphora analysis for the document; extraction means for setting the proper noun pair in the anaphora analyzed document and extracting the context information containing a plurality of words before and after the proper noun pair including the proper noun pair; and setting means for setting only a word that is registered in a vocabulary dictionary prepared in advance left from the extracted context information as the characteristic amount of the context information.