1. A storage system using flash memories comprising a storage controller and plural flash memory modules as storage media, each flash memory module comprising at least one flash memory chip, and a memory controller for leveling erase counts of blocks belonging to the flash memory chip, the storage controller: combining the plural flash memory modules into a first logical group, translating a first address used for accessing the flash memory modules belonging to the first logical group to a second address used for handling the first address in the storage controller, and combining the plural first logical groups into a second logical group.