The expansion of a mapping table with additional mapping information involved in the access to virtual volumes is prevented and so a search in the mapping information within a valid time period is ensured, thereby preventing degradation of system performance. A storage apparatus, which defines virtual volumes to which storage areas are dynamically allocated, has a data migration function to migrate data between the virtual volumes and real volumes. With the data migration function, the mapping information for the migrated data is deleted from the mapping table, and accordingly, the size of the mapping table can be limited.