PURPOSE: To attain a quick access to a cache memory by storing a correspondence table of real addresses stored in a cache memory and corresponding logical addresses into a cache control table and at the same time securing the access to the cache memory by the logical address.

CONSTITUTION: In a writing process an address save program 13 saves a logical address corresponding to a physical address of a magnetic disk and the physical address of the magnetic disk stored in a cache control table 9 is rewritten into the saved logical address when the writing mode is through. A logic disk driver 4 uses a hash table 10 by means of the logical address to retrieve the table 9. Then the driver 4 obtains the real address of a cache memory 8 and reads the requested data out of the memory 8. Thus it is possible to give an access to the memory 8 without converting a logical address once into a physical address of the magnetic disk.