1. An XPath evaluation method for evaluating a plurality of XPaths with respect to a predetermined data file by a computer, said method comprising the steps of: reading out a plurality of XPath expressions to be evaluated from a memory and generating a data structure that has a common part of said plurality of XPath expressions as a common node in a tree structure; inputting a data file to be an objective of processing from a memory and evaluating said data structure in node-by-node basis using said data structure with respect to said data file; and obtaining an evaluation result for each XPath expression by combining said evaluation results for each of nodes and storing the obtained evaluation result into a memory.