An apparatus for spectrometrically measuring an isotopic gas is adapted to determine concentrations of a plurality of component gases in a gaseous test sample by introducing the gaseous test sample into a cell, then measuring intensity of light transmitted through the gaseous test sample at wavelengths suitable for the respective component gases and processing data of the light intensity, characterized in that the cell receiving the gaseous test sample introduced therein is positioned in the light path between a light source and a photoreceptor, and a reference cell filled with a reference gas having no absorption at the wavelengths for measurement is disposed in a portion of the light path not occupied by the cell.
