Spectroscopic observations of the Na i D emission lines of prominences were made with the Domeless Solar Telescope in Hida Observatory. When active prominences are bright in the D2 emission line, the intensity ratio of D1 to D2 is found to deviate significantly from the theoretical ratio of the optically-thin case. On the other hand, the intensity ratio is close to the theoretical ratio for the most part of quiescent prominences. Furthermore, the full widths at half maximum intensity of the D2 emission line for active prominences become wider than those of the D1 line, as the intensity of the D2 line gets higher. These observed features clearly show that the emitting region of the Na i D lines is optically thick in some types of prominences. Non-LTE calculations were made by taking the ionization degree of hydrogen atoms and the thickness of the prominences and the electron temperature as free parameters. It is shown that the electron temperature of the emitting region of the Na i D lines should be as low as 4000 K for an explanation of the large optical thickness of the Na i D lines for active prominences. Brief discussions are included about the possible existence of low temperatures in active prominences.