1. A document processing apparatus in which structured documents are processed, comprising: a document type holding means for holding information of a plurality of document types; a document holding means for holding a plurality of documents each of which is composed according to any one of the document types held in the document type holding means; a document process designating means for designating a target document type as a target to be processed and process contents to be executed; a document type collating means for constructing finite-state automatons from a content model defining lower elements which can be included in an element of a document type based on the document types held in the document type holding means and for comparing said finite-state automaton of the target document type with finite-state automatons of document types other than the target document type, thereby obtaining related document types having a predetermined relationship with the target document type; and a document process executing means for executing the process instructed by the document process designating means on a document representing the target document type and a document of the related document type which are targets to be processed among the documents held by the document holding means.