In this paper, a representation method for structure information retrieval without schema and DTD of XML documents is presented. While existing researches have used DTD or schema to extract structure information, we extract structure information directly from XML documents. Especially the well-formed XML documents are only required to retrieve documents in the proposed method. Thus it can retrieve more general and various documents easily compared to the existing XML retrieval systems requiring DTD or schema. Experimental result indicates that the proposed method retrieves effectively structure information of XML documents independently to DTD information.