A syntax coverage percentage measuring system includes a BNF rule check table which has BNF data respectively corresponding to BNF syntax rules. A first file reading section reads each of test input files and carries out lexical analysis to data of each of the read test input files to classify into tokens. A first syntax analyzing section carries out syntax analysis to each of the tokens, and marks one of the BNF data of the BNF rule check table corresponding to the token. A coverage percentage output section acquires a total number of the BNF data and a number of the marked BNF data from the BNF rule check table, and calculates a coverage percentage based on the total number of the BNF (data and the number of the marked BNF data. An output section outputs the coverage percentage calculated by the coverage percentage output section.