Abstract: A data grouping means divides data items stored in a cache memory section into groups of data having different access patterns. The priority assigning means assigns an order of priorities to data items in each group that the priority assigning means manages according to an individual caching algorithm. The lowest priority determining means determines the lowest priority group when there is not enough unused memory space in the cache memory section and it is necessary to purge a data item. The data operating means purges the lowest priority data in the lowest priority group. Thus the groups of data having different access patterns can be cached effectively.