1. A structured document processing system for processing a structured document that is structurally represented and contains one or more document parts, the structured document processing system comprising: instruction providing means for providing an extraction instruction to extract a document part, a repetitive duplication instruction to duplicate a document part by a given number of times, or an insertion/substitution instruction to insert or substitute a document part to a corresponding document part by embedding the instruction in a structured document containing the document part; analyzing means for analyzing the structure of the structured document to generate a parse tree; instruction separating means for separating the instruction embedded in the structured document and the document part to retrieve the instruction, based on the parse tree generated by the analyzing means; document processing description generating means for merging and sorting the extraction instruction retrieved by the instruction separating means from a first structured document in which the extraction instruction is embedded, and the repetitive duplication instruction and/or insertion/substitution instruction retrieved by the instruction separating means from a second structured document in which the repetitive duplication instruction and/or insertion/substitution instruction is embedded to generate a document processing description having an instruction string for processing the first and second structured documents; extracting means for retrieving a first document part subject to the extraction instruction from the first structured document; repetitive duplication means for repeatedly duplicating a document part subject to the repetitive duplication instruction and an instruction provided to the document part by a given number of times; inserting/substituting means for inserting the first document part before or after a second document part subject to the insertion/substitution instruction or substituting the first document part for the second document part; and an interpreter for sequentially interpreting the document processing description and synthesizing document parts by using the extracting means, the repetitive duplication means, and/or the inserting/substituting means.