5. A structured data storage device comprising: a storage means for storing a structured data file having structured data into a first storage medium to be removably attached to the structured data storage device, and store 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; a detecting means for detecting whether or not the structured data file stored in the first storage medium has been updated by an external device; and an index information generating means for analyzing, when the detecting means detects that the stored structured data file has been updated, the updated structured data file, generate new index information relating to the structured data included in the updated structured data file, and update 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 means 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 means 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 means 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.
5. A structured data storage device comprising: a storage means for storing a structured data file having structured data into a first storage medium to be removably attached to the structured data storage device, and store 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; a detecting means for detecting whether or not the structured data file stored in the first storage medium has been updated by an external device; and an index information generating means for analyzing, when the detecting means detects that the stored structured data file has been updated, the updated structured data file, generate new index information relating to the structured data included in the updated structured data file, and update 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 means 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 means 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 means 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. 6. The structured data storage device according to claim 5 , further comprising a search means for searching the structured data based on the index information, in accordance with externally specified search information, wherein the search means: specifies a search element and content information of the search element in accordance with the externally specified search 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; and extracts, from the first index component, content information of at least one of the search elements, the content information being associated with the information which identifies the extracted reference element.