1. A natural language processing apparatus comprising: a result acquisition unit for acquiring a plurality of analysis results indicating parts of speech of morphemes contained in one or more common sentences, the analysis results obtained by analyzing the one or more common sentences using a plurality of types of morphological analyzers; a pattern acquisition unit for detecting a common segmentation point in the plurality of analysis results acquired by the result acquisition unit, extracting one or more parts of speech corresponding to a character string segmented at the common segmentation point from each of the analysis results, and acquiring a set of the extracted one or more parts of speech as a part-of-speech differing pattern; a storage unit for storing the part-of-speech differing pattern acquired by the pattern acquisition unit together with the number of appearances of the part-of-speech differing pattern; and a candidate specifying unit for extracting the part-of-speech differing pattern with the number of appearances being equal to or less than a predetermined threshold from the storage unit and specifying the character string corresponding to the extracted part-of-speech differing pattern as a character string containing a candidate for an unknown word.