Light scattering spectra of liquid and plastic phases of succinonitrile have been measured around the phase transition temperature for a wide frequency range using a Sandercock-type tandem Fabry-Perot interferometer and a double monochromator. Although the molecular structures in these two phases have been found similar, the spectral difference becomes prominent with decreasing frequency. Particularly, the onset of the difference lies in the so-called low-frequency modes (< 200 cm(-1)), implying that the cooperativity among various modes is essential for the manifestation of macroscopic nature.