1. A path index lookup method comprising: extracting a plurality of path information representing an information source from a document that is written in a predetermined language; receiving identification (id) information corresponding to the plurality of path information and index flag information indicating whether a respective piece of the plurality of path information has been stored in a storage unit, and storing path id information and the index flag information in a PathLookup table storage unit; receiving a source id and a target id of said respective piece of the plurality of path information and, if the index flag information corresponds to a predetermined value, storing the source id and the target id of said respective piece of the plurality of path information in an ExtendedEdge table storage unit; receiving a regular path expression used to retrieve information, searching the PathLookup table storage unit for path expression information that matches the regular path expression, and obtaining path id information corresponding to a path of the information to be retrieved; and searching the ExtendedEdge table storage unit for a source id and a target id of path id information that matches said path id information corresponding to the path of the information to be retrieved.