Abstract: An index creating unit divides text information into words and creates index information associated with schema identification information and identification information for each of the words divided. When the number of pieces of the index information stored in a file format exceeds a threshold, an index analyzing unit analyzes a distribution of a schema identification information group and an identification information group. When an analysis shows that more than a predetermined number of the schema identification information substantially match the schema identification information stored in a first rule storage unit, a first compressing unit compresses the schema identification information group using group identification information and in-group identification information stored in the first rule storage unit.