A gas analyzer system has been developed to analyze Hydrogen–Deuteride (HD) gas for producing frozen-spin polarized HD targets, which are used for hadron photoproduction experiments at SPring-8. Small amounts of ortho-H 2 and para-D 2 gas mixtures ( ∼ 0.01 % ) in a purified HD gas are a key to realize a frozen-spin polarized target. However, there was an intrinsic difficulty to measure these small mixtures in the HD gas with a quadrupole mass spectrometer (QMS) because D + and H 2D + produced from the ionization of HD molecules were misidentified as H 2 and D 2 molecules, respectively, and became backgrounds for the measurement of the H 2 and D 2 concentrations. In addition, the ortho-H 2 and para-D 2 are not distinguished from the para-H 2 and ortho-D 2, respectively, with the QMS. In order to obtain reliable concentrations of these gas mixtures in the HD gas, we produced a new gas analyzer system combining two independent measurements with a gas chromatograph and the QMS. Helium or neon gas was used as a carrier gas for the gas chromatograph which was cooled at ∼ 110 K . The para-H 2, ortho-H 2, HD, and D 2 are separated using the retention time of the gas chromatograph and the mass/charge ratio. Although the para-D 2 is not separated from the ortho-D 2, the total amount of the D 2 is measured without the H 2D + background. The ortho-H 2 concentration is also measured separately from the D + background. It is found that the new gas analyzer system can measure small concentrations of ∼ 0.01 % for the ortho-H 2 and D 2 with good S / N ratios.