The development of a real-time measurement system to determine the concentrations of combustion gases mixed with smoke particles in fire environments is an essential technical issue in the fire safety field. In this study, the absorption line for the J = 8–7 rotational transition at 708.9 GHz of hydrogen cyanide (HCN) produced by the combustion of silk is rapidly measured in a 1 m cell during repeat scans (seven times a minute) via terahertz spectroscopy. The obtained time profile of HCN concentrations agrees well with the time profile obtained via Fourier transform infrared (FT-IR) spectroscopy. The maximum concentrations derived via terahertz spectroscopy are consistent with those derived via FT-IR within an accuracy of 10%. The minimum sensitivity of HCN is 100 ppm as detected via terahertz spectroscopy. The repetition rate, the accuracy, and the sensitivity for the scans demonstrate the potential of terahertz spectroscopy to rapidly diagnose combustion gas flow concentrations in fire environments.