One previous study suggested that self-reflection positively affected decentering, but self-rumination negatively affected it. However, the study was cross-sectional, and temporal relationships between the variables were not sufficiently explained. In the present study, we conducted a two-wave longitudinal survey to elucidate the temporal association between self-reflection, self-rumination, and decentering. We found that self-reflection significantly interacted with self-rumination predicting Time 2 decentering after controlling for Time 1 decentering. The test of simple slopes indicated that self-reflection very weakly, but marginally significantly, buffered the negative effect of self-rumination on decentering. This result suggests that self-reflection contributes to maintaining a decentered perspective against facing negative self-discrepancy.