1. A structured data storage device configured to allow viewing or updating, from an external device, of structured data file stored in a storage medium built in the structured data storage device, comprising: a storage unit configured to store, 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; a detecting unit configured to detect whether or not the stored structured data file has been updated by the external device; and an index information generating unit configured to analyze, when the detecting unit 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 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 uniquely discriminates the reference element, information which identifies a structured data file having the structured data comprising the discriminated reference element, information which identifies the position of the discriminated reference element in the structured data included in the identified structured data file, information which identifies respective search elements positioned below the discriminated reference element, and content information of the identified search element, the second index component links together information which identifies respective search elements, content information of the identified search element, and information which uniquely discriminates a reference element having the identified search element below itself, the index information generating unit 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 unit links together, as the new first index component, information which uniquely discriminates the detected reference element, information which identifies the updated structured data file having the structured data comprising the discriminated reference element, information which identifies the position of the discriminated reference element in the structured data included in the identified structured data file, information which identifies the detected search element positioned below the discriminated reference element, and content information of the detected search element, and the index information generating unit links together, as the new second index component, information which identifies the detected search element, content information of the identified search element, and information which uniquely discriminates the detected reference element having the identified search element below itself.