4. A dialogue control system that uses an ontology that systematically expresses domain knowledge representing concepts and relationships between concepts to be used in questions, determines questions for a user and implements the questions, receives user answers to the questions, and controls a dialogue in accordance with the user answers, wherein the domain knowledge includes the concepts and relationships between concepts, and dialogue progress information that specifies movement relationships from concepts employed in questions to concepts to be employed in subsequent questions, and the dialogue control system includes: a dimension-classified ontology storage unit that stores at least one dimension-classified ontology, for each of dimensions for combinations of conditions, the dimension-classified ontologies differing in domain knowledge content; a basic ontology storage unit that stores a basic ontology including domain knowledge content for when there is no condition; a dimension priority storage unit that stores dimension priorities representing priorities of the respective dimensions; a dimension-classified ontology fetcher that, when a concept in the user answers is specified as a condition, fetches from the dimension-classified ontology storage unit one or more dimension-classified ontologies that meet the specified condition; a multidimensional ontology generator that overwrites the content of the fetched dimension-classified ontologies onto the basic ontology according to the dimension priorities of the dimension priority storage unit, in order from the dimension-classified ontology with the lowest dimension priority, and generates a multidimensional ontology to be used with the condition; and a dialogue controller that decides on a concept to be employed in a subsequent question on the basis of the dialogue progress information of the multidimensional ontology generated by the multidimensional ontology generator, and provides the question to the user.
6. The dialogue control system according to claim 4 , wherein the domain knowledge specifies at least check texts and link texts for relationships between concepts, the multidimensional ontology generator dynamically changes the check texts and link texts by generating multidimensional ontologies with concepts in the user answers as conditions, and the dialogue controller outputs the dynamically changed check texts and link texts to the user.