Differential management information that is information representing the difference between a logical volume and a sequential device group after a full backup is updated in response to an update of the logical volume. Partial data that are data representing the difference between the logical volume and sequential device group are specified by using the updated differential management information, information necessary for restoration that is necessary to restore the data group located in the updated logical volume by using the partial data is recorded in the sequential device group, and the partial data are backed up from the updated logical volume into the sequential device group.