STORAGE DEVICE

A storage device comprises plural memory units and a storage controller that controls the memory units as a RAID group. Each memory unit is provided with a nonvolatile semiconductor memory (e.g. flash memory) chip and a memory controller that compresses data and stores the compressed data into the nonvolatile semiconductor memory chips. The memory controller makes a logical memory area available to the storage controller. The storage controller divides the logical memory area into plural entries each of which is a logical memory area of a prescribed size, acquires from respective memory unit capacity information on the data capacity stored into the nonvolatile semiconductor memory, and exchanges data of entries between the semiconductor memory units on the basis of the capacity information.