For gas analysis systems based on terahertz time-domain spectroscopy (THz-TDS), relatively complex systems have been proposed to enhance the sensitivity to measure small amounts of gas. This paper proposes a simple system comprising a hollow optical fiber with an inner dielectric layer as a low-volume and long-path gas cell THz-TDS-based gas analysis system. The inner dielectric layer of the fiber not only reduces the transmission loss for THz waves, but also protects the metal layer of the fiber from reactive gases. The proposed dielectric-loaded fiber is fabricated by depositing a metal thin film on a polymer film tube. In a gas analysis experiment conducted using the fiber with a THz-TDS system, separating the absorption peaks of NH3 gas and water vapor initially proved to be difficult, although NH3 gas was successfully detected. However, by improving the frequency resolution via Fourier transform and introducing a longer hollow optical fiber, the two peaks were successfully separated and the system sensitivity improved.