1. An information search system comprising: conversion means 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 information search means of performing an information search by using the inquiry sentence generated by said conversion means and by referring to an ontology corresponding to the contents described in the inquiry sentence, wherein said information search means performs an information search by referring to the ontology relating to the description of said inquiry sentence generated by said conversion means and by setting, as a search condition, semantic information described according to the ontology; wherein said conversion means generates said inquiry sentence 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.