In this study we have identified the signals from the HDO isotope of water molecule in the emission spectra of the atmosphere recorded with a TIIS sensor along the zenith direction under clear sky conditions in December 1997 near Tokyo. Based on the algorithms available in the FIRE-ARMS software package we have developed a technique to determine the concentration ratio HDO/H2O in the troposphere from the TIIS spectra recorded in the range from 700 to 1500 cm-1 with the resolution of 0.15 cm-1. Thus obtained HDO/H2O concentration ratios agree with the characteristic values of this ratio that are being observed at these latitudes under similar conditions.