An erasure declaration-related write request is received. In cases where, in response to the erasure declaration-related write request, erasure-corresponding data elements which are data elements corresponding to an erasure target and which are stored in a storage area A in the first logical volume are overwritten with erasure data elements which are data elements signifying erasure at or after the snapshot acquisition time point, the storage area A is associated with a storage area B in which encrypted data elements corresponding to the erasure-corresponding data elements stored in storage area A are stored.