The passive open-path Fourier-Transform-Infrared system was implemented for the toxic gas monitoring. Noise Equivalent Differential Temperature(NEDT) was investigated as a system performance evaluation figure and analyzed numerically with the designed parameters. Calculated NEDT was compared with the experimental value in the wavelength region of $700{sim}1400cm^{-1}^$. The minimum detectable gas concentration was estimated from the obtained NEDT at the absorption wavelength of $SF_6$.