1. A database system comprising: a plurality of database servers each of which includes a database from which the same content can be searched and retrieved in response to a query; a front-end server that receives said query from a client terminal, and that poses said query to any of said database servers according to at least one predetermined rule; a management server that manages said rule with a rule management unit to be used by said front-end server wherein said servers and client terminals that issue a query are interconnected; and said management server further comprising: a log acquisition unit for acquiring a processed query log relevant to each database server; and a rule production unit for producing said rule according to a compatibility value of a query calculated using the acquired query log in a compatibility calculation unit; and said front-end server further comprising a query posing unit that uses the rule produced by said management server to pose the query.