a first storage system, coupled to a first client computer, having a first volume accessed by the first client computer, first snapshots of the first volume, and a first log information on the first snapshots; and

a second storage system, coupled to a second client computer and the first storage system, having a second volume accessed by the second client computer, second snapshots of the second volume, and a second log information on the second snapshots,

wherein the first storage system records creation timings of the first snapshot into the first log information, and transmits a part of the first snapshots to the second storage system,

wherein the second storage system stores a copy of the first volume in a past into the second volume by applying the part of the first snapshots which are transmitted, and controls a creation of the second snapshots for keeping data stored in the second volume at timings, each associated with one of timings of the applications of the part of the first snapshots, with recording the creation timings of the part of the first snapshots as timings of the second snapshots Into the second log information,

wherein after receiving a fail-back request, the first storage system identifies one of the first snapshots and one of the second snapshots based on the first log Information and the second log information, both are corresponded to a common timing based on the first log information and the second log information, roll-backs data recorded in the first volume to the common timing using the one of the first snapshots, and applies an another part of the second snapshots which timings are newer than the common timing, and

wherein after receiving a fail-back request, the first storage system deletes a certain part of the first snapshots which timings are newer than the common timing, and information on the certain part of the first snapshots from the first log information.