6. The natural language based service selection system according to claim 5 , wherein the automatic service selecting section further comprises a similar query detecting unit which searches a history query that contains the lost parameter in the current query as a similar query from the current user query history base when there is no parameter value extracted by the latest query detecting unit, and extracts the parameter value corresponding to the lost parameter in the current query from the similar query.
6. The natural language based service selection system according to claim 5 , wherein the automatic service selecting section further comprises a similar query detecting unit which searches a history query that contains the lost parameter in the current query as a similar query from the current user query history base when there is no parameter value extracted by the latest query detecting unit, and extracts the parameter value corresponding to the lost parameter in the current query from the similar query. 7. The natural language based service selection system according to claim 6 , wherein the similar query detecting unit retrieves the history query meeting the following conditions as a similar query from the current user query history base: a service type of the history query is identical to a service type to which the current query belongs; and a query parameter of the history query contains a query parameter of the semantically-analyzed query.