[en] To develop a real time and in-situ process gas analyzer for fusion fuel gas processing systems, application study of laser Raman spectroscopy was performed by measurement of hydrogen isotopes. Using an Ar ion type laser of which wavelength 488 nm, power 0.7 W, and single pass irradiation method, Raman spectra of hydrogen isotopes were measured and intensities of the Stokes rotational lines and Q-branch were quantitatively analyzed. The Stokes rotational lines at 587, 443 and 415 cm^-1 were selected as suitable ones for quantitative analysis of H₂, HD and D₂. Normalizing the Raman intensity of partial pressure H₂ as 100, relative Raman intensity ratio of H₂ : HD : D₂ was obtained as 100 : 58 : 47. The detection limit for hydrogen was estimated as 0.05 kPa in partial pressure and 500 ppm in concentration. But multiple pass method further improved the detection limit to 100 ppm. (author)