PURPOSE: To improve the hit rate of data, by allocating dynamically a tracking buffer to an external storage device, and decreasing the down-rate of a controller and the external storage device.

CONSTITUTION: A microprogram and a host access table and the like are stored in a control stoage 11. The tracking buffer is stored in a tracking buffer area 13. A host interface control circuit 16 performs communication with a processor. A timer 17 is used for externing a time stamp to the entry. A counte 18 counts the number of usage of a magnetic disc. A transfer control circuit 19 performs the automatic transfer control of data according to the command of the microprogram. A device interface control circuit 20 performs communication with devices.