The demand for private education in Korea is steadily increasing every year, and the participation rate of private education is increasing as the grade goes down. In order to empirically verify the effectiveness of private education, it is necessary to analyze through longitudinal data that has been mainly investigated over a long period of time. This study investigated the longitudinal changes in mathematics academic achievement and participation time in mathematics private education using longitudinal data from 2013 (4th grade in elementary school) to 2017 (2nd grade in middle school) of the Seoul Education Longitudinal Study. The students were divided into groups in which mathematics academic achievement changed similarly as the grade went up, and the effect of mathematics academic achievement was examined according to the change of participation time in private mathematics education for each group. As a result of the study, it was found that the participation time of private math education of all students continuously increased from the 5th grade of elementary school to the 2nd grade of middle school, and the participation time of private math education by group was different. In addition, the effect of private tutoring by group was different according to the group.