The paper proposes a pre-processing method for efficient processing of XML queries in information retrieval systems with a large amount of XML documents. For the pre-processing, we use a signature-based approach. In the conventional (flat document-based) information retrieval systems, user queries consist of keywords and boolean operators, and thus signatures are structured in a flat manner. However, in XML-based information retrieval systems, the user queries have the form of path query. Therefore, the flat signature cannot be effective for XML documents. In the paper, we propose a structured signature for XML documents. Through experiments, we evaluate the performance of the proposed method.