A storage system includes a primary volume and a first controller that stores data in a primary volume and stores content at a prescribed point in time in the form of differential data. The first controller generates generational management information corresponding to the differential data of the prescribed point in time and manages the generational management information. The storage system also includes a secondary volume configured to store backup data and a second controller configured to receive data sent from the first controller, store data to the secondary volume, store the generational management information, receive a request for a certain generation of the generations and provide data of the primary volume corresponding to the requested generation based on the request. The first controller determines to send differential data and the generational management information to the second controller when an amount of differential data reaches a prescribed amount.