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 thereof, 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: assigning a partial tree identifier inclusive of a tag identifier to the selected partial tree wherein the tag identifier includes a name of a tag corresponding to a root of said selected partial tree, a name of at least one format attribute of the tag, and a value of said at least one format attribute; arranging names of format attributes in a predetermined order in the tag identifier if said at least one format attribute of the tag includes two or more format attributes; and identifying a partial tree having a partial tree identifier identical to the partial tree identifier of the selected partial tree from a list of partial tree identifiers of partial trees that exist in the structured document after updating thereof.