There is provided a data processing system which, from a point of view of increasing process efficiency of the entire data processing system to exhibit a maximum effect, may assign an alias device to a logic device. The data processing system of this invention comprises a host processing device (10) and a storage control device (20) connected thereto via a communication means (50), where the storage control device (20) has a deciding means for deciding, at a time of processing an I/O request of a certain base device, whether there is a need to newly assign an alias device to the base device according to a use situation of a physical device (203). When assignment is decided as necessary, the host processing device (10) is notified of such via the communication means, and the host processing device (10) newly assigns an alias device to the base device when there is the notification from the storage control device (20).