Most of the existing studies on storing and searching XML documents effectively manipulate each XML document independently. Therefore, techniques for storing XML documents together that have similar meaning or structure are required for efficiency. Also, as a unified access method for various XML storage systems that have different storage forms, studies to integrate the DTD or XML schema of each storage system into one are required, because many XML documents do not have a particular DTD or XML schema, or XML documents can be written in various ways. Therefore, studies on the integration techniques for XML instances are needed. The XML integration technique can be used effectively in the case of constructing a data warehouse for heterogeneous XML storage systems. The proposed integration techniques remove the space duplicated for the same elements in XML documents. The proposed techniques significantly reduce the search time for general queries on the XML documents because it stores the related parts in XML documents close.