1. A method of generating a structured document for a structured document generating apparatus having at least an input/output device, a control unit, and a repository wherein a non-structured document not explicitly given the document structure and input from said input/output device is converted into a structured document explicitly given the document structure, in accordance with a document structure definition defining the document structure, said method comprising the steps of: modifying a given first document structure definition so as to match the document structure of said input non-structured document and generate a second document structure definition; by said control unit, generating a parsing rule used for performing a parsing process suitable for the document structure of said second document structure definition, by modifying marks constituting said second document structure definition and modifying said second document structure definition so as to make the positional order of said marks in one-to-one correspondence; in accordance with said generated parsing rule, generating a first structured document from said non-structured document; and in accordance with difference data between said first document structure definition and said second document structure definition, converting said generated first structured document into a format matching said first document structure definition to thereby generate a second structured document.