Data migration and data migration method

Second storage means according to the migration destination volume storage virtualization technology virtual volume mapping the migration source volume. The host system includes a host access path is switched from the access path to the migration source volume to the migration destination access path to the volume. Second storage means performs copy processing, copy processing based on the first compact in the first storage means with information contained in the allocation area with the virtual volume migration destination volume migration from the migration source volume migration source according to reduced volume data, and virtual volume with a copy of the data from the migration destination volume to the second storage means according to thin copy destination volume. Second memory means to copy destination volume and the migration destination virtual volume associated volume, instead of the source virtual volume.