The purpose of this study is to explore variables influencing teacher burnout using the teacher panel data from Seoul Education Longitudinal Study of 2020 (SELS 2020). Two-way ANOVA and LASSO regression analyses were applied, considering both school grade and years of teaching experience. The main findings are as follows: first, the results of the ANOVA revealed that both the main effect of teaching experience and the interaction effects between teaching experience and school grade were statistically significant. The average level of burnout among teachers with 20-25 years of experience was lower compared to that among new teachers and those with 5-15 years of experience (p<.001). Second, according to the results of the LASSO regression, relationship conflicts with students, colleagues, and parents were identified as the most influential variables that had positive effects on burnout. Third, when analyzed by school grade, constraints on teacher autonomy, parental interest or school demands regarding teaching led to teacher burnout in elementary schools, while fixed beliefs about intelligence, performance-based teacher evaluation, and the burden of student guidance and counseling triggered burnout in middle schools. In the analysis by teaching experience, conflict with parents was a major variable in the novice group and the 20-25 years experience group. Based on these results, this study suggested policy implications for preventing teacher burnout and establishing support systems.