A definition tool of the present invention declares columns which are concealed in external tables to a multi-database server. The multi-database server controls a decomposition and an execution of query messages such that the multi-database server does not perform an arithmetic using the concealed columns. The external database server encrypts the data of concealed columns in tables of query result tables. The multi-database server designates a range to be decrypted at the time of acquiring and integrating respective result tables. A client program acquires an integrated result table and decrypts the designated range. Since the data encrypted by the external database server is decrypted by and is referred to the client program, it becomes difficult for the multi-database server to refer to a content of the concealed columns.