1. A snapshot management apparatus for creating and managing a snapshot or snapshots of one or more generations, each snapshot being composed of an image of an operational volume at a specified time, wherein data from a host system is written to the operational volume and the data in the operational volume is read into the host system, the snapshot management apparatus comprising: a volume setting unit for setting the operational volume in a storage area provided by a first storage apparatus and setting a difference volume in a storage area provided by a second storage apparatus; a difference data saving unit for saving, in the difference volume, difference data that is the difference between the operational volume at the time of creation of the relevant snapshot and the operational volume in its current state, in response to data writing to the operational volume; and a snapshot managing unit for creating a management table that stores management information for the snapshots, and managing each snapshot according to the management table; wherein the snapshot managing unit creates the management table which is divided according to one or more snapshots; and if there is more than one management table, the second and any following management tables store specified inheritance information to inherit the content of the preceding management tables.