A system, a method, and a program for structured document derivation for generating derived structured documents of different representations having the same contents, facilitating the test for a structured document processor, and avoiding the vulnerability of the structured document processing. A monitor monitors a call from a structured document processing generator to a structured document processor to have a test structured document. A structured document derivation system generates derived structured documents of the test structured document. The derived structured documents are sent to a testing device for testing the structured document processor. A test analyzer determines whether or not each derivative pattern can be accepted. When the processing of a structured document is requested, the test analyzer analyzes the document. When the structured document matches an unacceptable pattern, the document is replaced with an acceptable one.