In order to search a file to be searched which includes records having fields allocated to each of a plurality of hierarchical levels and is constructed so that records having a same key character string in a field at a same hierarchical level are arranged in series, an index file is created by obtaining start position information and number information about records having a key character string contained in the hierarchical level of each node on the file to be searched, or a pointer to node management information of a lower hierarchical level, and by recording the information obtained for the nodes of all the hierarchical levels. Further, data is extracted from the file to be searched by using the record start position information, the record number information, and the pointer to the node management information of a lower hierarchical level recorded in the index file.