We developed the common schema conversion program for RDB schema into XML schema. Especially, we showed the schema constraints (such as PRIMARY KEY, CHECK, NOT NULL, UPDATE CASCADE ON DELETE, UNIQUE) can be converted. In the future research, we are going to develop the integration program of XML DB schema into the common schema. In addition, we plan to implement the common data manipulation API (for example, extension of the existing XQuery modules) to access the virtual databases and we are going to incorporate location transparency functions to this API.