In this paper, to automatically generate musical thumbnails that contain the main part of the original tune, we propose a new estimation method for identifying structure changes in stereo tunes based on localization information. The proposed method can estimate the main parts of a musical tune by analyzing the specific timing when localization information changes under the assumption that the changing time of the localization approximately corresponds to the timing of the musical structure change. We evaluate the effectiveness of the proposed method by objective and subjective assessments. The experimental results show that the proposed method is effective in automating musical structure analysis for generating musical thumbnails.