1. An information search system comprising: a data processor coupled to a memory and operable for executing instructions in the memory to perform steps of: inputting a natural language sentence and generating an inquiry sentence described in an ontology description language on the basis of the results of syntactic analysis and semantic analysis on the natural language sentence; and performing an information search by using the generated inquiry sentence and by referring to an ontology corresponding to the contents described in the inquiry sentence, wherein said performing step is performed by referring to the ontology relating to the description of said generated inquiry sentence and by setting, as a search condition, semantic information described according to the ontology; wherein said inquiry sentence is generated by examining a property to which a predicate of said input natural language sentence corresponds and a case frame to which the natural language sentence corresponds, on the basis of a case frame ontology dictionary in which are stored information indicating to which property in an ontology does a relationship among a predicate, a subject and an object in the natural language sentence correspond, and by describing the obtained property and case frame in said ontology description language.