Spectroscopic observations of the Nal D emission lines of prominences were made with the Domeless Solar Telescope in Hida Observatory. When active prominences are bright in the D 2 emission line, the intensity ratio of D~ to D 2 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 D 2 emission line for active prominences become wider than those of the D l line, as the intensity of the D 2 line gets higher. These observed features clearly show that the emitting region of the Nal 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 NaI D lines should be as low as 4000 K for an explanation of the large optical thickness of the NaI D lines for active prominences. Brief discussions are included about the possible existence of low temperatures in active prominences.