Abstract: The apparatus for processing Standard generalized markup language document generates a SGML conversion form file correlating a prototype file having a specific form with document type definition by a SGML conversion form generation module, and converts, in a case where a plain text document prepared using the specific form is converted to a SGML document, the prepared plain text document to the SGML document by referring to the SGML conversion form file in a SGML document generation module, while in a case the reverse conversion is executed, the converted SGML document is reversely converted to the plain text document by referring to the SGML conversion form file in a SGML document read-in module.