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.