1. A method of extracting information from a structured document, wherein the structured document is converted into a tree structure in order to identify a selected portion in the structured document after updating of the structured document, the selected portion being selected in advance from the structured document, and the selected portion corresponding to a selected partial tree, comprising the steps of: generating a tag identifier for a given tag of the structured document by combining a name of the given tag, a name of at least one format attribute of the given tag, and a value of said at least one format attribute, the tag identifier including the name of the given tag, the name of said at least one format attribute of the given tag, and the value of said at least one format attribute; assigning a partial tree identifier to the selected partial tree wherein the partial tree identifier includes the tag identifier of a tag corresponding to a root of said selected partial tree; arranging names of format attributes in a predetermined order in the tag identifier when said at least one format attribute of the tag includes two or more format attributes; storing the partial tree identifier and an identifier of the structured document as a pair in a storage unit; retrieving an updated structured document through a network in response to the identifier of the structured document stored in the storage unit; identifying a partial tree having a partial tree identifier identical to the partial tree identifier stored in the storage unit from a list of partial tree identifiers of partial trees which exist in the updated structured document; generating numerical value indexes which indicate respective sequential numbers of tag identifiers in the same hierarchical level of the tree structure; and combining a tag identifier and a numerical value index into a set, and connecting a plurality of sets of a tag identifier and a numerical value index in series from a root of the tree structure to the root of the selected partial tree, the combining and connecting producing the partial tree identifier, wherein a plurality of tags in the structured document have the name of the tag and the name of said at least one format attribute identical to respective names of the selected partial tree.