parsing means for parsing a structured document to be stored in accordance with a structured document storage request made by a client;

structured document storing means for storing parsed data obtained by parsing the structured document in the structured document database, the parsed data being stored in the structured document database as a partial tree of a tree structure of a single virtual structured document, wherein the structured document database includes a structure information table including a plurality of entries corresponding to the nodes of the single virtual structured document, each of the entries storing a parent node ID, a child node ID, a preceding-sibling node ID, and a following-sibling node ID of a node of the single virtual structured document, and a plurality of node information blocks storing node information for one or more nodes of the single virtual structured document, the structured document storing means including:

(a) first storing means for acquiring node information of each node of the structured document from the parsed data obtained by parsing the structured document, and storing the node information in a node information block of the structured document database;

(b) second storing means for extracting a parent node ID, a child node ID, a preceding-sibling node ID, and a following-sibling node ID of each of the nodes from the parsed data obtained by parsing the structured document, and storing structure information for indicating the extracted parent node ID, child node ID, preceding-sibling node ID, and following-sibling node ID in an entry of the structure information table of the structured document database;

(c) parent-child relationship setting means for setting the structure information so as to have a parent-child relationship between a root node of the tree structure of the single virtual structured document and an uppermost node of the parsed data obtained by parsing the structured document, wherein the parent-child relationship setting means stores, in an entry of the structure information table which corresponds to the uppermost node, a node ID of the root node as the parent node ID of the uppermost node; and

(d) sibling relationship setting means for, if another parsed data corresponding to another structured document is stored in the structured document database by the structured document storing means, setting the structure information so as to have a sibling relationship between the uppermost node of the parsed data obtained by parsing the structured document and an uppermost node of the another parsed data, wherein the sibling relationship setting means stores, in an entry of the structure information table which corresponds to the uppermost node of the another parsed data, a node ID of the uppermost node of the parsed data obtained by parsing the structured document as the following-sibling node ID of the uppermost node of the another parsed data, and stores, in the entry of the structure information table which corresponds to the uppermost node of the structure information table which corresponds to the uppermost node of the parsed data obtained by parsing the structured document, a node ID of the uppermost node of the another parsed data as the preceding-sibling node ID of the uppermost node of the parsed data obtained by parsing the structured document;

structured document acquiring means for acquiring parsed data corresponding to data of a structured document, which meets to a structured document acquisition request made by the client, from the structured document database; and

request processing means for returning the parsed data acquired by the structured document acquiring means to the client.