A data structure of index information for retrieving pair character strings on a computer at high speed is provided. A method of retrieving a pair character strings appearing in close proximity of each other in a document using the index information at high speed is also provided. Bits of a suffix array of reference document data are rearranged, thereby creating index information LSA localizable, or usable as an index for a subregion of the document. Through use of this, a process of dichotomizing a region, where the entire document is designated as an initial region, is repeated and positions of index information for a query character string in the reference document data are gradually detailed. The distance between the pair is evaluated and candidates are narrowed down. Finally, positions where the pair character strings occur in close proximity of each other are identified.