We report the first in situ NMR observation of H-2(2) adsorbed inside [Cu-3(btc)(2)] (HKUST-1; btc: benzenetricarboxylate) at ambient temperature and pressure. Hydrogen adsorption behavior was investigated by hydrogen sorption analysis and in situ solid-state H-2 NMR under controlled H-2(2) gas pressure. Although HKUST-1 adsorbed very little amount of H-2 at 303 K, we could successfully observe the H-2(2) signal inside HKUST-1 in the NMR spectra. Furthermore, the NMR signal demonstrated that HKUST-1 interacts with H-2 molecules even at ambient temperature and hydrogen pressure.