13. A structured data storage method in a structured data storage device which allows viewing or updating, from an external device, of structured data file stored in a storage medium built in the structured data storage device, comprising the steps of: storing, into the storage medium, the structured data file having structured data, and an index file having index information for use to search the structured data; detecting whether or not the stored structured data file has been updated by the 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 stored index file 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 included 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.
13. A structured data storage method in a structured data storage device which allows viewing or updating, from an external device, of structured data file stored in a storage medium built in the structured data storage device, comprising the steps of: storing, into the storage medium, the structured data file having structured data, and an index file having index information for use to search the structured data; detecting whether or not the stored structured data file has been updated by the 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 stored index file 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 included 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. 16. The structured data storage method according to claim 13 , further comprising an update step of updating elements of the structured data, in accordance with externally specified update information, wherein the update step: specifies a search element of a candidate to be updated, content information of the search element of the candidate to be updated, element selection condition which specifies the search element of the candidate to be updated, in accordance with the externally specified update 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; replaces the content of the element which is positioned below the read reference element and satisfies the element selection condition, in accordance with the contents to be updated; and causes the storage step to store again the structured data file with the elements already updated, into the storage medium, and causes the index information generating step to analyze the structured data file with the elements already updated to update the index information.