To study application of laser Raman spectroscopy for fusion fuel gas analysis by an in situ method, methane (CH4) and tritium (T2) mixed gases were measured. In the mixed gases, hydrogen isotope exchange reactions were induced by beta decay, and various isotopic hydrogens and methanes were produced. Spectral peaks of v1 and v3 bands were detected individually for CH4 and four tritiated methanes. The v1 bands between 1700–1900 cm−1 were selected as suitable ones for quantitative analysis. After mixing T2 and CH4 gases, while large amounts of tritiated methanes were produced as time lapsed, the equilibrium state was not reached by the time 1000 h had passed. It was presumed that the isotope exchange reactions were very slow compared to mixed gases of just hydrogen isotopes.