  We report on the IR spectroscopic studies in both reflection (50-900 cm^{-1})
and transmission (900-3000 cm^{-1}) mode of the vibration spectrum of the
cuprous oxide. A detailed analysis based on a comparison of the temperature
dependences of the absorption band at 1125 cm^{-1} and of IR and Raman active
fundamental vibrations results in assignment of the former to a biphonon.
