PURPOSE: To leave the decision related to the data fetching to an external device and therefore to reduce the load of a central processor, by adding the proper information to the head of the data which is read out of a magnetic disk and transferred.

CONSTITUTION: When an optional data of a magnetic disk device 4 is sent to an external device 6, the proper information of the data is read into a stacker 11 from a main storage device and delivered to the device 6 in preference to the data which is read into a data buffer 10 from the device 4. The proper information is added to the head of the data sent from a multiplexer 12. The device 6 detects the flow of the data accordant to the data request on the basis of the proper information to fetch inside the desired data only.