A storage unit system includes a control apparatus (1010) having a unit for reading memory data from a plurality of storage units (1021 to 1024) before increase into a memory (1012) of the control apparatus, a preparing unit for preparing parity data newly from the memory data read in the memory, a rearrangement unit for dispersing transfer data from a processor (1001) read in the memory and the newly prepared parity data to be written into a plurality of storage units (1021 to 1025) after the increase to perform arrangement of data, a memory unit (1035) for storing a write position on the way of the rearrangement of data, a comparison unit for comparing an access position for an access request from the processor with the write position, and a determining unit for determining a data dispersed pattern used in a data access from the processor on the basis of a comparison result of the comparison means, whereby the storage unit can be increased individually with a unit of one storage unit and dynamically without stop of the system.