The director fluctuation and associated optical flickering effect in a nematic 4-pentyl-4 ' -cyanobiphenyl (5CB) liquid crystal has been studied by time-resolved spectroellipsometry (TRSE). The comparison of the results of total reflection TRSE (surface dynamics) with those of transmission TRSE (bulk dynamics) indicates that the flickering effect has its origin in director fluctuation near the substrate surface caused by a change in internal electric field due to ion migration. The shape and intensity of the flickering response curve shows a characteristic frequency dependence, indicating that several different ionic species contribute to director fluctuation. It is also demonstrated that the observed line shape can be reproduced by a numerical simulation that assumes ion migration.