To perform processing at a high speed to identify some of the structure type definitions in a given definition set to which a received XML document matches. There are provided a decision rule generation section which generates a decision rule for determining to which one of a plurality of DTDs an XML document matches on the basis of unique elements of the DTDs, and a DTD determination section which determines to which one of the plurality of DTDs the XML document to be processed matches on the basis of the decision rule generated by the decision rule generation section.