Green House Gas (GHGs) emission reduction has gained large prominence globally due to the climate change. Nitrous Oxide (N₂O) emission from transportation has significant share on global warming. Therefore, an instrument for sensing ultra-low level of N₂O is a key global demand. In this study, development of an instrument based on the Quantum Cascade Laser (QCL) has been attempted for measuring ultra-low level N₂O in automobile exhaust gas sampled in a sample storage bag. The QCL can emit coherent lights in a mid-infrared (Mid-IR) region where N₂O shows strong absorption peak. This absorption peak can be detected by an MCT (Mercury Cadmium Tellurium) type photovoltaic detector. The optics configuration used in this study can give a superfine resolution of the Mid-IR spectrum such as 0.002 cm-₁ in the target wavelength band. Therefore, utilizing this spectrometer, measurement of ultra-low level N₂O is possible without interference of co-existing gases. The instrument developed has been evaluated using actual engine exhaust to confirm the interference of co-existing gases, measurement accuracy and repeatability. Very low detection limit (less than 5 ppb) has been achieved. The instrument exhibits excellent linearity, therefore can be calibrated easily with standard gas. The rise time (T₁₀-T₉₀) is shorter than 5 seconds.