The purpose of this study is to investigate the performance of the student-selective curriculum and the influencing factors through a case study of an autonomous private high school that operates the curriculum, which is the basic principle of the high school credit system. To this end, this study interviewed school members and analyzed it using the Modified Grounded Theory Approach. The results of the study are as follows. The school offered various subjects for students to choose autonomously depending on their career paths and aptitudes, which improved their self-direction, learning motivation, and suitability for major resulting in excellent university admissions. The factors to make the achievements are a wide range of subject choices, a diploma system to inform course completion conditions, and the curriculum center to develop and operate the curriculum. In addition, school management based on a clear educational philosophy, collaborative school culture with the support of school corporation, and the college admission system consistent with the school curriculum contributed to the performance.