1. A speech dialogue system comprising a processor and one or more memories, the processor configured to: estimate an attribute of an input proper name by using information stored in a database; estimate the attribute of the input proper name by using information obtained from the web; calculate integrated reliability of estimation for each of possible attributes obtained from the estimation based on the database and the web, by integrating first reliability and second reliability, the first reliability being reliability of estimation based on the information stored in the database, the second reliability being reliability of estimation based on the information obtained from the web; and generate a response sentence to an input utterance on the basis of the integrated reliability of the possible attributes.