On the first storage device side, when data is written in a first volume, a journal of such data is transmitted to a second storage device; on the second storage device side, a snapshot of a second volume and the respective journals are stored; on the second storage device side, the first volume is rebuilt at a rebuilding time according to a rebuilding order; on the second storage device side, data of an address rewritten from the rebuilding time onward is transmitted as difference data to the first storage device; and on the first storage device side, the first volume is rebuilt at the rebuilding time based on the difference data.