Abstract: A natural language including Japanese, Korean, Chinese, etc., is suitably processed when an undefined word which is not registered in a dictionary appears in an input character train. When the undefined word appears, a semantic analysis processor decides a semantic concept of the undefined word by using a case dictionary. Further, by using a vocabulary hierarchy dictionary, which registers vocabularies in a hierarchy structure, the semantic analysis processor extracts a group of vocabularies (synonyms) corresponding to a lower concept of the semantic concept of the undefined word. The natural language processing can be achieved practically and efficiently with dictionaries of a limited storage capacity.