1. A structured-document search apparatus for searching a plurality of structured documents stored in a database for a document structure in a structured document corresponding to first structure-designating information, in response to a search request containing the first structure-designating information and issued from a client terminal, the first structure-designating information designating an arbitrary document structure including tags, comprising: structure-template-storing means for storing a plurality of structure templates as templates of tag structures in relation to identifiers assigned to the structure templates, the structure templates corresponding to document structures included in the structured documents stored in the database; a synonym dictionary which stores synonym group identifiers indicating synonym groups, and tags belonging to the synonym groups, said each synonym group being a set of tags having a common concept; synonym search means for search processing for searching the synonym dictionary for synonym group identifiers indicating synonym groups to which the tags included in the first structure-designating information belong; structure-designating-information generation means for generating, based on a search result of the synonym search means for search processing, second structure-designating information in which the tags included in the first structure-designating information are replaced with the respective synonym group identifiers indicating the synonym groups to which the tags belong; synonym-structure-template-storing means for storing synonym structure templates in which all tags included in the structure templates stored in the structure-template-storing means are replaced with the synonym group identifiers indicating the synonym groups to which said all tags belong; synonym-structure-template-searching means for searching the synonym-structure-template-storing means for a synonym structure template corresponding to the second structure-designating information; and acquisition means for acquiring, from the database, a document structure in a structured document indicated by the structure template searched for by the synonym-structure-template-searching means.