1. A storage system coupled to another storage system and managing both current data and data of a certain point of time in the past for n generations (n being an integer of 2 or more), the storage system comprising: a controller; and at least one storage device; wherein the controller comprises acquiring, from the other storage system, at least a data of generation n of a certain point of time and a data of generation n−1 at a point of time in the past than generation n; determining whether there is any difference data between the data of generation n and the data of generation n−1; and generating a difference information of data of generation n−1 with respect to generation n based on the determination result, and storing the same in the storage device.