To enable a user to perform, by using an ontology, an information search utilizing a meaning given to information on a network by Semantic Web without being required to perform complicated operations for forming an inquiry sentence in conformity with the ontology.In a stage before a search engine for performing an information search by using an ontology is provided an information search supporting system having a morpheme analysis section, a syntactic and semantic analysis section, and a conversion execution section which converts a natural language sentence on which syntactic analysis and semantic analysis have been performed into an inquiry sentence described in an ontology description language by referring to a case frame ontology dictionary in which are stored information indicating to which property in an ontology does the relationship among a predicate, a subject and an object in the natural language sentence correspond and the case frame of the natural language sentence in the property. A natural language sentence is input and an inquiry sentence described in the ontology language is formed on the basis of the results of syntactic analysis and semantic analysis on the natural language sentence and is provided to the search engine.