1. A snapshot management apparatus for creating and managing a plurality of snapshots of one or more generations, each snapshot being comprised 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 first and second difference volumes in a storage area provided by a second storage apparatus; a difference data saving unit for saving, in one of the difference volumes, 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 means for permitting continued operation of snapshot operation if one of the first or second difference volumes fails, said means including a snapshot managing unit for creating a plurality of management tables in a sequence of at least a first management table and a second management table, each of which is divided into blocks to store management information for the snapshots in accordance with the creation time of snapshots corresponding to the management table, and managing each snapshot according to the corresponding management table, wherein the first management table corresponds to the first difference volume and the second management table corresponds to the second difference volume, wherein the first management table stores management information for a first group of snapshots between a first creation time and a second creation time, and wherein the second management table stores management information for a second group of snapshots with creation times between the second creation time and a third creation time, subsequent to the second creation time, wherein the second management table stores specified inheritance information to inherit the content of the first management table; wherein the inheritance information stores a block address of the first difference volume in which difference data corresponding to a last block address of a last snapshot of the first management table is stored, wherein the volume setting unit sets the first and second difference volumes in the storage area provided by the second storage apparatus, and the difference saving unit saves the difference data of each snapshot in the corresponding difference volume, and the snapshot managing unit creates the first and second management tables associated with the respective first and second difference volumes, and wherein the snapshot managing unit sends each management table to its corresponding second storage apparatus and stores it in the corresponding difference volume.