The functions of a mainframe environment are expanded by leveraging the functions of an open environment. A second storage of an open system externally connected to a first storage of a mainframe system comprises a second main volume of an open environment generated in association with a main logical device of the second storage, and a second sub volume of an open environment generated in association with a sub logical device of the second storage; the first storage comprises a first main volume of a mainframe environment generated in association with the main logical device of the second storage, and a first sub volume of a mainframe environment generated in association with the sub logical device of the second storage; when the first storage receives a data processing request from a host, the first storage reflects the processing request in the second storage and completes the processing; and when the first storage receives an execution request of a prescribed function, the first storage causes the second storage to execute the function.