In a storage system with a server apparatus having at least one CPU, a memory, at least one network interface, and at least one storage interface, and, at least one disk connected to the storage interface, the server apparatus has a snapshot management part which maintains and manages snapshot of the disk apparatus, the disk apparatus has an operational volume which performs ordinary read/write and a differential volume which stores differential data of snapshot of the operational volume, the snapshot management part stores data on the operational volume in the differential volume, provides a preceding copy area to store its location information, and when free area on the differential volume becomes insufficient discards the stored location information and uses the preceding copy area as an free area.