Questions and answers associated with each other are stored in a document storage section. A clustering section classifies the answers in the document storage section into clusters based on feature vectors of the answers. When a natural language question is input by the user, a database retrieval/updating section retrieves a question similar to the input question, and presents answers associated with the retrieved question together for each cluster to the user or an expert. In addition, the database retrieval/updating section automatically updates the document storage section based on an answer selected as most appropriate by the user or the expert if selected, or based on an answer newly input by the expert if no appropriate answer is available. The natural language answer input by the expert is presented to the user as it is.