To elucidate the mechanism of the color-deepening phenomenon in aged red rice samples, a partial structure of proanthocyanidin, (+)-catechin-(4 → 8)-dimer (1), was synthesized as a model compound and subjected to chemical oxidation. HPLC analysis of oxidized 1 revealed new peaks which were isolated; among these, four compounds (2–5) having formed intramolecular bonds between the D and B rings were determined. These compounds and their derivatives in red rice extracts were identified by multiple reaction monitoring mass spectrometry. The UV–Vis spectra of 1–5 were recorded to clarify the correlation between the color-deepening effect and their chemical structures. Hence, the spectral absorption maxima between 300 and 500 nm, corresponding to the oxidation products, increase. Therefore, the color-deepening phenomenon of red rice is thought to proceed via the formation of intramolecular bonds in proanthocyanidin by oxidation.