an address comparator stores an address of data read out by a host system . also , a buffer reads out the data from a memory and stores the data . if an address of data which is expected to be newly read out by the host system is included in addresses which have already been stored in the address comparator , the host system 1 newly reads out the data from the buffer , not from the memory . as a result , it is possible to eliminate or lessen the possibility of unintentional rewriting of data which is likely to be caused due to repeated readout of data .