We have reported the Raman scattering and infrared absorption results on a t 2g orbital ordered Ca 2 RuO 4 . At 10K, a strong and clear peak was observed in Raman scattering near 1360cm −1 with x′x′ geometry. In contrast to optic phonon modes, the peak does not show any frequency shift but rapidly decreases with increasing temperature. In addition, the peak is not observed in infrared absorption measurement. By comparing the previous Raman scattering results for several transition metal oxides, we have discussed the possible origins and ambiguities of the intriguing peak in Ca 2 RuO 4 .