An index file for use in search processing employing a search engine, retains information that indicates a correspondence of a keyword to its positional information. The index file comprises a key file that includes a list of pointers to character strings. The character strings are included in document files that are stored in a document database and to positional information, by document area where a character string in a document file appears and a POS file. The POS file includes a list of positional information containing information that specifies a document file where a character string exists and information that specifies a position of the character string in a document file regarding the character strings in the key file.