A second-level cache device stores part of registration information of data for a first-level cache device in a second-level cache-tag unit in association with registration information in a second-level-cache data unit, and stores the registration information of data for the first-level cache device in a first-level cache-tag copying unit. A coherency maintaining processor maintains coherency between the first-level cache device and the second-level cache device based on the information stored in the second-level cache-tag unit and the first-level cache-tag copying unit.