A highly advantageous storage system and a snapshot management method. In the storage system, write target data sent from a host computer to a first storage controller is duplicated and stored in the first storage controller and also in a second storage controller; when the first storage controller receives a request from the host computer to create a snapshot in the second storage controller, it suspends transferring differential data to the second storage controller, creates a new first snapshot, and transfers differential data for the created first snapshot together with the untransferred differential data for a previous first snapshot for which transferring had been suspended, to the second storage controller.