A gas sensing technique on a fire site was developed. An output of is-TPG in a necessary low frequency for gas sensing controlled was obtained by optimizing the seeder so far. A reflection spectrum of an architectural material that used N₂O gas that was the model gas of CO gas generated at a fire was successfully obtained, and a realizability of remote spectrum sensing in the THz range was showed experimentally. An attenuation rate of the THz-wave when the long distance was propagated under the atmospheric pressure was measured experimentally, and an error margin with the calculation was within 10 %. A calculation result was able to endure practical use in the THz range to which the accuracy of the calculation was doubted.