Attenuated total reflection (ATR) spectroscopy in the terahertz (THz) region was used to measure the spectra of water, aqueous glycine solution, and glycine powder between 0.5 and 12THz. By subtracting the absorption spectrum of water, we obtained clear absorption peaks for dissolved glycine at 2.1, 5.2, and 9.3THz. In addition, we observed that the frequencies of these absorption peaks differed from the peaks for glycine powder. This indicates that the low-frequency vibrational mode in glycine molecules changes with the hydrogen bonding of glycine to H₂O molecules.