1. A language conversion apparatus comprising: a storage device that stores syntax definition information for defining a plurality of syntaxes for converting a business rule statement, which is written in a natural language and composed of one or more syntax elements, into a Drools Rule Language sentence on a syntax basis, and conversion information for converting an element variable included in the syntax element or syntax elements of the business rule statement into an element variable of the Drools Rule Language sentence; a syntax analysis device for judging which one of the plurality of syntaxes defined by the syntax definition information matches a syntax of the business rule statement, by comparing a character string of the business rule statement with a character string of the syntax element constituting each syntax defined by the syntax definition information; a conversion device that converts the business rule statement into the Drools Rule Language sentence by converting the element variable of the business rule statement into the element variable of the Drools Rule Language sentence with reference to the conversion information and converting the syntax element other than the element variable of the business rule statement into a syntax element of the Drools Rule Language sentence having the syntax which has been judged to match the syntax of the business rule statement, wherein the syntax definition information is classified into patterns so that the business rule statement can be converted into the Drools Rule Language sentence by means of a combination of a plurality of types of basic syntaxes; and an interface connected to the apparatus, which outputs the converted Drools Rule Language sentence.