A controller in a storage system has chunk statistical information to be information including an overlap prevention effect value of each stored chunk data. For each storage target chunk data to be each chunk data obtained by dividing a data block of a storage target, the controller executes an overlap search to search the stored chunk data matched with the storage target chunk data in an overlap search range to be a part of a plurality of stored chunk data shown by the chunk statistical information and a chunk data group specified from the chunk statistical information and stores the storage target chunk data in the storage device, when the matched stored chunk data is not discovered in the overlap search.