XML is a data-oriented language to represent all types of documents including web documents. By means of the advent of XML-based document generation tools and grow of proprietary XML documents using those tools and translation from legacy data to XML documents at an accelerating pace, we have been gotten a large amount of differently-structured XML documents. Therefore, it is more and more important to retrieve the right documents from the document set. But, previous works on XML have mainly focused on the storage and retrieval methods for a large XML document or XML documents had a same DTD. And, researches that supported the structural difference did not efficiently process path queries on the document set. To resolve the problem, we suggested a new inverted index mechanism using RDBMS and proved it outperformed the previous works. And especially, as it showed the higher efficiency in indirect containment relationship, we argues that the index structure is fit for the differently-structured XML document set.