Method of dividing data to be stored in storage device into data fragments; recording the data by using configurations of divided data fragments; judging whether identical data fragments exist in data fragments; when it is judged that identical data fragments exist, storing one of the identical data fragments in storage area of the storage device, and generating and recording data-fragment attribute information indicating an attribute unique to the data fragment stored; upon receipt of request to read data stored in the storage area of the storage device, acquiring the configurations of the data fragments forming the read-target data, reading the corresponding data fragments from the storage area of the storage device, and restoring the data; acquiring and coupling the recorded data fragments to generate concatenation target data targeted for judgment on whether chunk concatenation is possible or not, and detecting whether the concatenation target data has a repeated data pattern.