A computer receives a retrieval request with a retrieval character string, adds control information to a region corresponding to code information included in a management region used for extending encoded data when an extension result of the code information matches a first character of the retrieval character string when the encoded data to be retrieved is sequentially extended in response to the retrieval request, and performs collation processing between the retrieval character string and character string information corresponding to data of the extension result of the code information when the control information is added to a partial region corresponding to code information to be extended in the extension.