XML has become widely accepted for both representation and exchange of data over the Internet. This situation urges digital library systems to deal with XML documents as well as typical text documents. A standard query language is very helpful for interoperability among digital library systems over the Internet. XQuery is a forthcoming standard for querying XML data. In this paper we propose an XQuery Engine that can be used as an XQuery processing module in digital library systems that support XML documents. Our system has the following useful aspects. First, any user interface that generates XQuery is able to access any digital library systems embedding our XQuery Engine. Second, we define Primitive Operation Trees (POTs) so that they can become a standard interface between an XQuery Engine and an Information Retrieval Engine. Third, some query optimizations over POTs can be done in the XQuery Engine so that better searching performance is expected.