A storage resource is effectively utilized in migration of a snapshot. First de-duplication efficiency which is de-duplication efficiency of snapshot data, and second de-duplication efficiency which is de-duplication efficiency of snapshot entities created on the basis of the snapshot data are calculated and compared with each other. Based on a result of the comparison as well as a first preservation period set for the snapshot in the migration source and a second preservation period set for the snapshot in the migration destination, one of the following methods is selected: a first method of migrating an snapshot image, a second method of migrating snapshot data to an archive storage system, and a third method of migrating data materialized from a first differential volume, a second differential volume, and a switch snapshot.