The purpose of this study is to develop a core competency diagnostic tool. As for the contents of the study, the core competency diagnostic questions were developed and the reliability and validity of the core competency diagnostic questions were analyzed. The research results are as follows. First, 504 diagnostic questions for core competency were produced. Second, university professors and related experts developed 216 preliminary questions out of 504 diagnostic questions. Third, the preliminary questions were revised and supplemented by conducting a facial validity assessment of the preliminary questions for graduates with a doctoral degree in education. Fourth, as a result of analyzing the reliability and validity of the core competency diagnostic tool, it was found that there were no problems with reliability and validity.