Upon addition to the memory devices, above configurations enable the distribution of the data among the existing memory devices and the added memory device without restructuring of the data and thus make it possible to carry out the distribution of the data to the added memory device in a short period of time.

The memory devices have the lives based on the amounts of writing and the control unit determines the moved data volume for the movement from each of the existing memory devices to the added memory device, based on the life of the existing memory device in the first data distribution processing. Thus, the remaining lives of the existing devices may be leveled.

The control unit determines the timing of the start of the second data distribution processing, based on the life difference among the memory devices. Thus, the remaining lives of the existing devices and the added device may be leveled.

In the second data distribution processing, the data volume of the redundant data with respect to the user data is decreased. Thus, the overall data volume may be reduced, so that the lives of the memory devices may be extended.

The data is stored in a log structure scheme. In the second data distribution processing, the control unit does not change the number of pieces of the data in the parity group that is stored before the start of the second data distribution processing but changes the number of pieces of the data in the parity group that is stored after the start of the second data distribution processing, to the second number. Thus, distribution control that is suitable for the log structure scheme is enabled.