PURPOSE: To decrease the writing frequency to a disk by inhibiting the writing operations to the disk in case the entries here the write command acceptance bits are set are kept within a prescribed number.

CONSTITUTION: At the time of a disk write instruction is given to a disk controller 30 from a host device 10, a directory memory 32 is searched for check whether the data on the areas designated in a disk10 hits a cache memory 31 or not. If the data hits the memory 31, the registering operation is carried out to the memory 31 as long as the disk write instruction is specified. Here it is checked whether the number of blocks where the writing operations are not finished yet to the disk 20 is larger than N or not. If not, a magnetic disk controller 30 writes the write data given from the device 10 to the memory 31 only and not to the disk 20.