PURPOSE: To provide a cache system for achieving a high hit ratio through a minimum cache capacity.

CONSTITUTION: In the case that access is made by an address Ai+1, b(Ai+1<Ai+1, b<Ai+2), since designated storage contents di+1, b belong to a block Di+1, and is stored in the block M2 of a cache 104, the contents di+1, b are read out of the memory block M2 of the cache memory 104. Further, since no block Di+2 for following it is present on the cache, data Di-1 are expelled from the block M0 of a main memory 102 and the data Di+2 are read into the memory M0 from the main memory 102 instead of it. Thus, the data blocks Di, Di+2 before and behind the data block Di+1 made access this item come to be preserved in the cahce memory 104.