A cache system includes a data storage which stores the data, which a memory supplies, in memory regions that are identified by identification numbers. A management data storage stores a relation between the identification numbers and the addresses of data items stored or being stored in the data storage and stores validation information indicating whether the data storage stores the data items. A refill-processing unit designates a memory region in which the data of a new address supplied from a processor is to be stored and requests the memory to output the data of the new address. An inspection unit receives the identification number of the memory region designated, determines whether the validation information about the identification number of the memory region designated is valid, and requests the data storage to output the data stored in the memory region designated when the validation information is valid.