Terahertz spectra were obtained for polylactide, and analyzed by numerical calculations based on the density functional theory. An absorption band was observed at around 50 cm-1, which becomes larger when the THz electric field is parallel to the sample drawing direction. An absorption peak with a similar directional dependence appears at around 40 cm-1 in the simulation, which is due to normal-mode molecular vibrations parallel to the molecular helix. Since the 50-cm-1 absorption decreases with an increase in crystallinity and the permittivity shows a step-like downward change, the molecular vibrations responsible for the 50 cm-1 band seems to be subjected to strong damping in amorphous regions. On the other hand, another absorption peak at 65 cm-1 with a positive dependence on the sample's crystallinity is likely to be due to some lattice vibration, since it does not appear in the calculations that only deal with intramolecular vibration.