17. A structured data storage method in a structured data storage device which stores structured data file in a storage medium, comprising the steps of: storing the structured data file having structured data into a first storage medium to be removably attached to the structured data storage device, and storing an index file having index information for use to search the structured data into a second storage medium built in the structured data storage device; detecting whether or not the structured data file stored in the first storage medium has been updated by an external device; and analyzing, when the detection step detects that the stored structured data file has been updated, the updated structured data file, generating new index information relating to the structured data included in the updated structured data file, and updating the index file stored in the second storage medium using the new index information, wherein the structured data has a plurality of data units identically configured with hierarchically structured elements, each data unit comprises a reference element positioned at the top of the respective data units, and one or more search elements positioned below the reference element, the index information comprises a first index component and a second index component, the first index component links together information which identifies the reference element, information which identifies the structured data file having the structured data comprising the identified reference element, information which identifies the position of the identified reference element in the structured data included in the identified structured data file, information which identifies respective search elements positioned below the identified reference element, and content information of the respective search elements, the second index component links together information which identifies respective search elements, content information of the respective search elements, and information which identifies a reference element having the respective search elements, the index information generating step detects a reference element included in the structured data in the updated structured data file, analyzes the updated structured data file by detecting a search element positioned below the detected reference element, and generates the new index information comprising a new first index component and a new second index component, the index information generating step links together, as the new first index component, information which identifies the detected reference element, information which identifies the updated structured data file having the structured data comprising the identified reference element, information which identifies the position of the identified reference element in the structured data included in the identified structured data file, information which identifies the detected search element positioned below the identified reference element, and content information of the detected search element, and the index information generating step links together, as the new second index component, information which identifies the detected search element, content information of the respective search elements, and information which identifies the detected reference element having the respective search elements.
17. A structured data storage method in a structured data storage device which stores structured data file in a storage medium, comprising the steps of: storing the structured data file having structured data into a first storage medium to be removably attached to the structured data storage device, and storing an index file having index information for use to search the structured data into a second storage medium built in the structured data storage device; detecting whether or not the structured data file stored in the first storage medium has been updated by an external device; and analyzing, when the detection step detects that the stored structured data file has been updated, the updated structured data file, generating new index information relating to the structured data included in the updated structured data file, and updating the index file stored in the second storage medium using the new index information, wherein the structured data has a plurality of data units identically configured with hierarchically structured elements, each data unit comprises a reference element positioned at the top of the respective data units, and one or more search elements positioned below the reference element, the index information comprises a first index component and a second index component, the first index component links together information which identifies the reference element, information which identifies the structured data file having the structured data comprising the identified reference element, information which identifies the position of the identified reference element in the structured data included in the identified structured data file, information which identifies respective search elements positioned below the identified reference element, and content information of the respective search elements, the second index component links together information which identifies respective search elements, content information of the respective search elements, and information which identifies a reference element having the respective search elements, the index information generating step detects a reference element included in the structured data in the updated structured data file, analyzes the updated structured data file by detecting a search element positioned below the detected reference element, and generates the new index information comprising a new first index component and a new second index component, the index information generating step links together, as the new first index component, information which identifies the detected reference element, information which identifies the updated structured data file having the structured data comprising the identified reference element, information which identifies the position of the identified reference element in the structured data included in the identified structured data file, information which identifies the detected search element positioned below the identified reference element, and content information of the detected search element, and the index information generating step links together, as the new second index component, information which identifies the detected search element, content information of the respective search elements, and information which identifies the detected reference element having the respective search elements. 19. The structured data storage method according to claim 17 , further comprising a deletion step of deleting elements of the structured data, in accordance with externally specified elements-to-be-deleted information, wherein the deletion step: specifies a search element of a candidate for deletion, and content information of the search element of the candidate for deletion, in accordance with the externally specified elements-to-be-deleted information; extracts, from the second index component, a pair of information which identifies the specified search element and content information of the specified search element; extracts, from the second index component, information which is associated with the extracted pair and identifies the reference element; extracts, from the first index component, information which identifies the structured data file, the information being associated with the information which identifies the extracted reference element, and information which identifies the position of the reference element; reads the reference element from the structured data file identified by the information which identifies the extracted structured data file and the information which identifies the position of the extracted reference element; deletes at least one of the elements positioned below the read reference element; and causes the storage step to store again the structured data file with the elements already deleted, into the first storage medium, and causes the index information generating step to analyze the structured data file with the elements already deleted to update the index information.