1. An ontology processing device for processing an ontology, comprising: a data input device; a memory; a processor; and a computer program with instructions that, when said processor executes said instructions, is configured as a structuralizing device which corrects a structure of ontology in a prescribed form created from a set of instance data containing a combination of a subject, a property, and an object expressed with a character string; wherein the structuralizing device comprises a necessity degree judging device which judges whether each of the properties contained in the ontology is an essential property or unessential property for a concept that is defined within the ontology and related to each of the properties according to statistical features of the objects contained in the set of the instance data, and corrects the structure of the ontology regarding the corresponding properties according to results of the judgments, and wherein the necessity degree judging device changes the property that is judged as unessential to a property having an inverted definition range and an inverted value range.